Re: If Statement with Declaration
On Monday, 24 July 2017 at 13:12:17 UTC, Nick Treleaven wrote: static if (is(E == enum; alias V)) { // this inserts "V" V v; } The test comes first as it logically should, as the alias is not being used until the following statement. Hmm, it can be used in the test: is(Abc U : U*) is(AA A : A[B], B : int) is(AA T : T[U], U : const char[]) So the alias part would come first instead: is(alias U; Abc : U*) is(alias A; AA : A[B], B : int) is(alias T; AA : T[U], U : const char[]) Note: The spec page needs to use CamelCase for types (abc -> Abc, bar -> Bar), especially as the construct is difficult to read anyway. Maybe I'll do this soon. (I think other parts of the spec contravene this too).
Re: If Statement with Declaration
On Monday, 24 July 2017 at 13:12:17 UTC, Nick Treleaven wrote: On Friday, 21 July 2017 at 21:50:02 UTC, Johan Engelen wrote: static if (is(E V == enum)) { // this inserts "V" V v; } ``` Yes, but this is a bit different as the goal is to avoid repeating V Sigh. Repeating E. (unintentional ironic mistake)
Re: If Statement with Declaration
On Friday, 21 July 2017 at 21:50:02 UTC, Johan Engelen wrote: We do have a construct like that already: ``` static if (is(E V == enum)) { // this inserts "V" V v; } ``` Yes, but this is a bit different as the goal is to avoid repeating V (as it may be complex). Repeating a local variable name (using e.g. `with...if`) isn't a problem as the variable should have a short identifier. With the above `is` expression it's unavoidable to have a test and a declaration combined. That said, the syntax could be much clearer: static if (is(E == enum; alias V)) { // this inserts "V" V v; } This syntax clearly separates the test from the declaration, and makes the declaration obvious due to the alias keyword. The test comes first as it logically should, as the alias is not being used until the following statement. It extends to the other `is` expression forms: is(T; alias A) is(T : U; alias A) is(T : U!V, U, V; alias A) is(T == U!V, U, V; alias A) (http://dlang.org/spec/expression.html#IsExpression) Whenever I want to use the identifier variant I always end up checking the spec. When I see it in code I usually have to stop what I was thinking about to parse the `is` expression.
Re: If Statement with Declaration
On Friday, 21 July 2017 at 21:32:48 UTC, Andrei Alexandrescu wrote: with (auto r = makeMeARange) if (!r.empty) with (auto x = r.front) { ... } Andrei I'm being real nitpicky, but this in particular just seems like a slightly worse way to write with (auto r = makeMeARange) if (!r.empty) { auto x = r.front; ... } But yeah, it's cool construct that lends itself to pretty neat chaining.
Re: If Statement with Declaration
On Friday, July 21, 2017 17:32:48 Andrei Alexandrescu via Digitalmars-d wrote: > On 07/19/2017 09:30 AM, sontung wrote: > [snip] > > This post: > http://forum.dlang.org/post/vfjlpvpwuyfqoljvp...@forum.dlang.org seems > to be identical to one on November 3, 2016: > http://forum.dlang.org/post/dejodpslmjdovstdi...@forum.dlang.org. > > Hat tip for persistence! > > Regarding the feature itself: it seems to be fancied by the new > languages, and C++ added it, too. I must be old school because I don't > see much benefit in it, and don't quite find it natural. It's bizarre > even lexically: "If the following ... oh wait let me insert some stuff > ... as I was saying, if the following condition happens..." Conversely, > I find the let/letrec syntax in functional languages a bit more convivial. I would have thought that it was a fairly obvious extension given how for loops work (especially since you can already declare variables in if statements; you just can't do it and then use something else for the condition), and it's less verbose than doing something with with, though I suppose that getting with to work like this would be better than nothing. It does seem unnecessarily verbose in comparison though. - Jonathan M Davis
Re: If Statement with Declaration
On Friday, 21 July 2017 at 21:32:48 UTC, Andrei Alexandrescu wrote: It's bizarre even lexically: "If the following ... oh wait let me insert some stuff ... as I was saying, if the following condition happens..." (excuse me for muddying the waters) We do have a construct like that already: ``` static if (is(E V == enum)) { // this inserts "V" V v; } ``` -Johan
Re: If Statement with Declaration
On 07/19/2017 09:30 AM, sontung wrote: [snip] This post: http://forum.dlang.org/post/vfjlpvpwuyfqoljvp...@forum.dlang.org seems to be identical to one on November 3, 2016: http://forum.dlang.org/post/dejodpslmjdovstdi...@forum.dlang.org. Hat tip for persistence! Regarding the feature itself: it seems to be fancied by the new languages, and C++ added it, too. I must be old school because I don't see much benefit in it, and don't quite find it natural. It's bizarre even lexically: "If the following ... oh wait let me insert some stuff ... as I was saying, if the following condition happens..." Conversely, I find the let/letrec syntax in functional languages a bit more convivial. The suggested enhancement of "with" seems to sit well on the page: with (auto x = fun()) if (x > 0) { ... } and offers some additional flexibility such as: if (!r.empty) with (auto x = r.front) { ... } and also "for free" variations such as: while (!r.empty) with (auto x = r.front) { ... } and with (auto r = makeMeARange()) if (!r.empty) { ... } Of course there's always the opportunity to go bananas :o). with (auto r = makeMeARange) if (!r.empty) with (auto x = r.front) { ... } Andrei
Re: If Statement with Declaration
On Thursday, 20 July 2017 at 14:18:48 UTC, Jack Stouffer wrote: On Thursday, 20 July 2017 at 14:05:36 UTC, Iakh wrote: It is not about reduce number of lines. It is about binding related things in one statement. Even so, it's already been shown in this thread that the same effect can be achieved via a block statement (doing exactly what it was designed for) This decreases readability by splitting up parts that (the programmer wants to) semantically belong together. or with a for loop. Which is a hack decreasing readability, because it works opposite to what one generally expects when reading a looping control structure. It's an small increase in terseness for a decrease in readability and an increase in complexity. W.r.t to the `with` solution only: It's a noticeable increase in readability for a minor increase in complexity.
Re: If Statement with Declaration
On Thursday, 20 July 2017 at 14:05:36 UTC, Iakh wrote: It is not about reduce number of lines. It is about binding related things in one statement. Even so, it's already been shown in this thread that the same effect can be achieved via a block statement (doing exactly what it was designed for) or with a for loop. It's an small increase in terseness for a decrease in readability and an increase in complexity.
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 15:41:18 UTC, Jack Stouffer wrote: On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: Thoughts on this sort of feature? To be frank, I don't think that helping the programmer reduce the line count in their program by one line is worth further complicating the language. It is not about reduce number of lines. It is about binding related things in one statement.
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 15:31:08 UTC, ag0aep6g wrote: On 07/19/2017 03:30 PM, sontung wrote: So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I'd prefer a new variant of `with`: with (int i = someFunc()) if (i >= 0) { // use i } It's slightly more verbose, but the meaning is clearer (arguable). It extends automatically to other control structures like `switch`. I wouldn't have this new `with (declaration)` have the magic lookup rules of the existing `with (expression)`. It would be a simpler tool that I'd probably use more than the existing `with`. I like "with" variant. Very mach like haskells "where". I believe it would be cool with expressions. Even can emulate named function arguments with (const skip_comments = false, const skip_empty_line = true) auto diff_result = diff(textA, textB, skip_comments, skip_empty_line);
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 15:31:08 UTC, ag0aep6g wrote: On 07/19/2017 03:30 PM, sontung wrote: So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I'd prefer a new variant of `with`: with (int i = someFunc()) if (i >= 0) { // use i } It'd be nice to have either of these available in D, though I'd prefer the `with` one, since - `with` is currently not widely used - usually only for things like `switch (var) with (EnumName) {...}`; this would make `with` pull its own weight (so to speak) - I think it is way easier to read
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: Thoughts on this sort of feature? This doesn't enable anything new, and breaks readability conventions.
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 20:42:33 UTC, Steven Schveighoffer wrote: I remember reading a discussion about using with statements to do this earlier as well, but I can't find it. -Steve I don't think this is the discussion you're talking about, but this does bring DIP 1005 to mind: https://github.com/dlang/DIPs/blob/master/DIPs/DIP1005.md Personally, I'm in favor of `with` for both variable declarations and imports. It's pretty intuitive semantically.
Re: If Statement with Declaration
I thought I remembered reading about this. But apparently, it was *exactly* this. https://forum.dlang.org/post/dejodpslmjdovstdi...@forum.dlang.org And this is, ... well I guess I continue to have the same ideas :) https://forum.dlang.org/post/oknvgb$2nr0$1...@digitalmars.com https://forum.dlang.org/post/nvi4t8$1750$1...@digitalmars.com I guess we should see another thread like this in another 8 months? I remember reading a discussion about using with statements to do this earlier as well, but I can't find it. -Steve
Re: If Statement with Declaration
On Wednesday, July 19, 2017 1:30:56 PM MDT sontung via Digitalmars-d wrote: > So I was thinking of a way of extending if statements that have > declarations. The following being as example of the current use > of if statements with declarations: > > if(int* weDontPollute = someFunc()) > { > // use weDontPollute > } > > That's great and all, but it only works by checking if the > variable evaluates to true or false. Which is fine for a pointer > but otherwise useless for anything else, like integers where zero > is usually valid input (index for array). So currently the only > way to do something like this in the language, that i've found, > is to use a for statement. > > for(int i = someFunc(); i >= 0;) > { > // use i > > break; > } > > Not that ideal to use a for statement. It makes it hard to read > and if the break condition isn't there it might very well be an > infinite loop. So I was thinking of some sort of syntax like this: > > if(int i = someFunc(); i >= 0) > { > // use i > } > Thoughts on this sort of feature? I've wanted this for years, but based on previous conversations on the topic, I've assumed that we're never getting it. It sure wouldn't hurt my feelings though if someone put forth the time and effort to write a DIP for it. Maybe they could get it accepted. I don't know. - Jonathan M Davis
Re: If Statement with Declaration
On 7/19/17 11:47 AM, Jonathan Marler wrote: On Wednesday, 19 July 2017 at 15:39:02 UTC, Steven Schveighoffer wrote: On 7/19/17 9:30 AM, sontung wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I really like the idea. Only thing I don't like is the possibility for abuse/confusion/errors: if(int i = someFunc(); j >= 0) // typo, or weird relationship, or just intentional obfuscation? It reminds me a bit of why we got rid of the comma operator. This is why I've liked suggestions in the past like: if((int i = foo()) >= 0) That is, you want to use 'if' on an expression while saving the expression, but the if is only looking at a property of that expression. Note this makes if(arr) (the correct meaning, that is ;) much more palatable: if((auto x = getArray()).length) Don't get me wrong, if this syntax is what gets this idea in, I'm fine with it. One possibility is to require usage of the declared variable in the condition. I respectfully disagree with this. I recall many times where I want to declare variables that should be limited to the scope of the conditional block but aren't used in the condition itself, i.e { auto a = ...; auto b = ...; while(something) { // use a and b } } This is different. The variable exist outside the scope of the loop. This is more like a for-loop. Arguably, for-loops are better suited for this, and already support it: for(auto a = ..., b = ...; something;) An if statement runs once. There isn't an "exists for all the loops" for an if statement. So it's clean and neat to declare them in one place or the other. Only if you need to use it for the condition does the declaration have to be in the condition expression. But arguably, for loops can be (and have been) abused, so this isn't exactly new territory. Like I said, I'm OK with the proposal if that's what gets it done. -Steve
Re: If Statement with Declaration
On Wed, Jul 19, 2017 at 01:30:56PM +, sontung via Digitalmars-d wrote: [...] > if(int i = someFunc(); i >= 0) > { > // use i > } > Thoughts on this sort of feature? I've been wanting such a feature for a long time now. Though I'd propose a different syntax: if ((int i = someFunc()) >= 0) { ... } T -- I've been around long enough to have seen an endless parade of magic new techniques du jour, most of which purport to remove the necessity of thought about your programming problem. In the end they wind up contributing one or two pieces to the collective wisdom, and fade away in the rearview mirror. -- Walter Bright
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 15:31:08 UTC, ag0aep6g wrote: I'd prefer a new variant of `with`: Looks much cleaner to me.
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 16:49:38 UTC, Jonathan Marler wrote: This would automatically make it work with any statement: with(auto x = 0) if(x) { doSomething } with(auto x = 0) while(x) { doSomething } Could also do multiple with statements with(auto x = 0) with(auto y = 0) if(check(x, y)) { doSomething } Yes. That's exactly the idea. Only with this synax it's worth it.
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 16:13:28 UTC, Swoorup Joshi wrote: On Wednesday, 19 July 2017 at 15:31:08 UTC, ag0aep6g wrote: On 07/19/2017 03:30 PM, sontung wrote: So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I'd prefer a new variant of `with`: with (int i = someFunc()) if (i >= 0) { // use i } It's slightly more verbose, but the meaning is clearer (arguable). It extends automatically to other control structures like `switch`. I wouldn't have this new `with (declaration)` have the magic lookup rules of the existing `with (expression)`. It would be a simpler tool that I'd probably use more than the existing `with`. I really prefer this over if. This just made sense, just at a glance Could also make 'with' behave like 'if', where it could apply to a block or just a single statement, i.e. // 'if' examples if(x) doSomething if(x) { doSomething } // 'with' examples with(auto x = 0) doSomething with(auto x = 0) { doSomething } This would automatically make it work with any statement: with(auto x = 0) if(x) { doSomething } with(auto x = 0) while(x) { doSomething } Could also do multiple with statements with(auto x = 0) with(auto y = 0) if(check(x, y)) { doSomething }
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 16:13:28 UTC, Swoorup Joshi wrote: On Wednesday, 19 July 2017 at 15:31:08 UTC, ag0aep6g wrote: On 07/19/2017 03:30 PM, sontung wrote: So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I'd prefer a new variant of `with`: with (int i = someFunc()) if (i >= 0) { // use i } It's slightly more verbose, but the meaning is clearer (arguable). It extends automatically to other control structures like `switch`. I wouldn't have this new `with (declaration)` have the magic lookup rules of the existing `with (expression)`. It would be a simpler tool that I'd probably use more than the existing `with`. I really prefer this over if. This just made sense, just at a glance Me too. Would also allow with(char c = foo()) switch(x) { // use c } so no need to switch over the new declared variable, but still beeing able to use it there
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 15:31:08 UTC, ag0aep6g wrote: On 07/19/2017 03:30 PM, sontung wrote: So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I'd prefer a new variant of `with`: with (int i = someFunc()) if (i >= 0) { // use i } It's slightly more verbose, but the meaning is clearer (arguable). It extends automatically to other control structures like `switch`. I wouldn't have this new `with (declaration)` have the magic lookup rules of the existing `with (expression)`. It would be a simpler tool that I'd probably use more than the existing `with`. I really prefer this over if. This just made sense, just at a glance
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 15:41:18 UTC, Jack Stouffer wrote: On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: Thoughts on this sort of feature? To be frank, I don't think that helping the programmer reduce the line count in their program by one line is worth further complicating the language. As Steve mentioned, it's three lines and apart and it makes a huge difference if you have to review a large codebase. After all, syntactic sugar is one of the reasons why Python is so popular.
Re: If Statement with Declaration
On 7/19/17 11:41 AM, Jack Stouffer wrote: On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: Thoughts on this sort of feature? To be frank, I don't think that helping the programmer reduce the line count in their program by one line is worth further complicating the language. It's 3 lines. One for the extra scope, one for the declaration, and one for the closing scope. Hm... just had an idea: auto propertyCheck(string condition, T)(T val) { static struct Result { T _value; opCast(B: bool)() { mixin("return _value " ~ condition ~ ";"); } alias _value this; } return Result(val); } if(auto x = someFunc().propertyCheck!" >= 0") { // use x as if it was the result of someFunc() } Has some drawbacks, for instance you may want to use the true booleanness of whatever T is inside the function. -Steve
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 15:39:02 UTC, Steven Schveighoffer wrote: On 7/19/17 9:30 AM, sontung wrote: [...] I really like the idea. Only thing I don't like is the possibility for abuse/confusion/errors: if(int i = someFunc(); j >= 0) // typo, or weird relationship, or just intentional obfuscation? It reminds me a bit of why we got rid of the comma operator. This is why I've liked suggestions in the past like: if((int i = foo()) >= 0) That is, you want to use 'if' on an expression while saving the expression, but the if is only looking at a property of that expression. Note this makes if(arr) (the correct meaning, that is ;) much more palatable: if((auto x = getArray()).length) Don't get me wrong, if this syntax is what gets this idea in, I'm fine with it. One possibility is to require usage of the declared variable in the condition. -Steve Now that I think about an extension of the comma makes more sense: if (int i=foo(), i<42) ... It is exactly the idea that we are trying to convey here (except that, deprecated comma aside, it doesn't work because of scope issues: i in i<42 is considered undefined). Given that the comma will never die and that it conveys exactly the idea I think think it should be prefered to the semi-colon.
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 15:39:02 UTC, Steven Schveighoffer wrote: On 7/19/17 9:30 AM, sontung wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I really like the idea. Only thing I don't like is the possibility for abuse/confusion/errors: if(int i = someFunc(); j >= 0) // typo, or weird relationship, or just intentional obfuscation? It reminds me a bit of why we got rid of the comma operator. This is why I've liked suggestions in the past like: if((int i = foo()) >= 0) That is, you want to use 'if' on an expression while saving the expression, but the if is only looking at a property of that expression. Note this makes if(arr) (the correct meaning, that is ;) much more palatable: if((auto x = getArray()).length) Don't get me wrong, if this syntax is what gets this idea in, I'm fine with it. One possibility is to require usage of the declared variable in the condition. -Steve I respectfully disagree with this. I recall many times where I want to declare variables that should be limited to the scope of the conditional block but aren't used in the condition itself, i.e { auto a = ...; auto b = ...; while(something) { // use a and b } } I imagine this feature allowing it to be rewritten as: while(auto a = ...; auto b = ...; something) { // use a and b }
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: Thoughts on this sort of feature? To be frank, I don't think that helping the programmer reduce the line count in their program by one line is worth further complicating the language.
Re: If Statement with Declaration
On 7/19/17 9:30 AM, sontung wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I really like the idea. Only thing I don't like is the possibility for abuse/confusion/errors: if(int i = someFunc(); j >= 0) // typo, or weird relationship, or just intentional obfuscation? It reminds me a bit of why we got rid of the comma operator. This is why I've liked suggestions in the past like: if((int i = foo()) >= 0) That is, you want to use 'if' on an expression while saving the expression, but the if is only looking at a property of that expression. Note this makes if(arr) (the correct meaning, that is ;) much more palatable: if((auto x = getArray()).length) Don't get me wrong, if this syntax is what gets this idea in, I'm fine with it. One possibility is to require usage of the declared variable in the condition. -Steve
Re: If Statement with Declaration
On 07/19/2017 03:30 PM, sontung wrote: So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I'd prefer a new variant of `with`: with (int i = someFunc()) if (i >= 0) { // use i } It's slightly more verbose, but the meaning is clearer (arguable). It extends automatically to other control structures like `switch`. I wouldn't have this new `with (declaration)` have the magic lookup rules of the existing `with (expression)`. It would be a simpler tool that I'd probably use more than the existing `with`.
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 14:46:31 UTC, Andrea Fontana wrote: On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: Thoughts on this sort of feature? I think I read this topic a couple of times here, and it went nowhere... Andrea Well someone must take the initiative and submit a DIP for this feature. I miss it a lot too and would try to help this DIP.
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? Achieving the same semantics as your example today would look like this { int i = someFunc(); if(i >= 0) { // use i } } The disadvantage being that it creates an extra level of nesting. I can see how this extra scope can discourage people from properly scoping their variables and this feature would eliminate the need for the extra scope. Also note that this example shows how this could be implemented using "syntax lowering". if( ; ; ... ;
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: Thoughts on this sort of feature? I think I read this topic a couple of times here, and it went nowhere... Andrea
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: [...] I like it for C++17 and I like it for D. Atila
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 14:26:52 UTC, Dukc wrote: On Wednesday, 19 July 2017 at 13:37:50 UTC, Adam D. Ruppe wrote: I like it. Me too. I think this should also apply to switch and with statements. Perhaps while statements too. Right, C++17 has it also for a switch: ``` If init-statement is used, the switch statement is equivalent to { init_statement switch ( condition ) statement } ``` http://en.cppreference.com/w/cpp/language/switch
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? This is included in C++17: http://en.cppreference.com/w/cpp/language/if ``` int demo() { if (auto it = m.find(10); it != m.end()) { return it->size(); } if (char buf[10]; std::fgets(buf, 10, stdin)) { m[0] += buf; } ... ```
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 13:37:50 UTC, Adam D. Ruppe wrote: I like it. Me too. I think this should also apply to switch and with statements. Perhaps while statements too.
Re: If Statement with Declaration
On Wednesday, 19 July 2017 at 13:30:56 UTC, sontung wrote: Thoughts on this sort of feature? I like it.
If Statement with Declaration
So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature?
Re: If Statement with Declaration
On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? Hi, i refactored some templates[1] i had, so that you can provide your own predicate functions. Variant 1 eager version: auto when(alias pred, alias fun, T)(T type = T.init) Variant 2 a lazy version, uses two templates: auto when(alias pred, T)(T type = T.init) auto call(alias fun, T)(T type) check/modify the examples found here [1] http://melpon.org/wandbox/permlink/g7V0gj4V7SGPjwqL *OT is dpaste broken (cant seem to create new pastes) or is it just me?
Re: If Statement with Declaration
On Sunday, 6 November 2016 at 05:07:10 UTC, Andrei Alexandrescu wrote: // possible future D if ((auto variable = fun()) != 42) { ... } Defining a variable in an expression wouldn't be allowed everywhere (but might be contemplated later as an possibility, which is a nice thing about this syntax). I like it but it would have to require the parantheses, or you could get ambiguities like: if (auto variable = noParensGetSomeT == true) { // is variable of type T or bool, if T can be implicitly cast? } A more approachable thing to do is allow variable definitions in switch statements: switch (auto x = fun() { ... } It is surprising that doesn't work, which is a good argument in favor of the feature (removal of an undue limitation, rule of least astonishment etc). This I can get behind, would start using it right away. Andrei
Re: If Statement with Declaration
On Sunday, 6 November 2016 at 05:07:10 UTC, Andrei Alexandrescu wrote: if (auto variable = fun(); variable != 42) { ... } Why does the word "variable" need to appear twice? It seems simpler to allow punctuation around existing syntax: // possible future D if ((auto variable = fun()) != 42) { ... } Avoiding stuttering is nice, but maybe it could be made a bit clearer if declarations were allowed for ref arguments and we used a named function: alias let = (ref v) => v; if (let(auto variable = fun()) != 42) {...} The same feature allows tuple unpacking: produceTuple.unpack(auto str, int i); C# has argument declarations. Just an idea. I personally like the C++ syntax too as local variable names are usually short and it avoids brackets.
Re: If Statement with Declaration
On Sunday, 6 November 2016 at 05:07:10 UTC, Andrei Alexandrescu wrote: The declaration with "if" seems to be a recent fashion. I've first seen it in Go and now C++17 took a shine to it - http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0305r0.html. A DIP would do good to cite that related work. It seems a low impact feature. Also, the Go/C++ syntaxes seem suboptimal to me because they are stuttering: if variable := fun(); variable != 42 { ... } or (C++): if (auto variable = fun(); variable != 42) { ... } Why does the word "variable" need to appear twice? It seems simpler to allow punctuation around existing syntax: // possible future D if ((auto variable = fun()) != 42) { ... } Defining a variable in an expression wouldn't be allowed everywhere (but might be contemplated later as an possibility, which is a nice thing about this syntax). Andrei I remember an old suggestion/DIP allowing 'with' statements to introduce/declare symbols/variables. Might be a cleaner extension to existing language: with(auto x = f()) if(foo(x)) {} else with(auto r = root(t)) if(leaf(r) || blah < bar) {}
Re: If Statement with Declaration
On Sunday, 6 November 2016 at 05:07:10 UTC, Andrei Alexandrescu wrote: A more approachable thing to do is allow variable definitions in switch statements: switch (auto x = fun() { ... } It is surprising that doesn't work, which is a good argument in favor of the feature (removal of an undue limitation, rule of least astonishment etc). https://issues.dlang.org/show_bug.cgi?id=11070 lol
Re: If Statement with Declaration
On 11/5/16 3:52 PM, Jonathan M Davis via Digitalmars-d wrote: Ultimately though, the biggest hurdle is that someone needs to create a DIP for it that strongly argues its case with real world examples of how it would improve code (preferably with code from Phobos and code.dlang.org), and without a really well-written DIP it's going to be dead in the water The declaration with "if" seems to be a recent fashion. I've first seen it in Go and now C++17 took a shine to it - http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0305r0.html. A DIP would do good to cite that related work. It seems a low impact feature. Also, the Go/C++ syntaxes seem suboptimal to me because they are stuttering: if variable := fun(); variable != 42 { ... } or (C++): if (auto variable = fun(); variable != 42) { ... } Why does the word "variable" need to appear twice? It seems simpler to allow punctuation around existing syntax: // possible future D if ((auto variable = fun()) != 42) { ... } Defining a variable in an expression wouldn't be allowed everywhere (but might be contemplated later as an possibility, which is a nice thing about this syntax). A more approachable thing to do is allow variable definitions in switch statements: switch (auto x = fun() { ... } It is surprising that doesn't work, which is a good argument in favor of the feature (removal of an undue limitation, rule of least astonishment etc). Andrei
Re: If Statement with Declaration
On Saturday, 5 November 2016 at 19:52:05 UTC, Jonathan M Davis wrote: and more importantly, I don't know if Walter or Andrei would think that it's worth it, and they're the ones who need to be convinced. this addition looks like needless overcomplication even for me. and i'm usually collecting feature for aliced without any formalities. ;-)
Re: If Statement with Declaration
On Thursday, November 03, 2016 22:29:34 Jerry via Digitalmars-d wrote: > if(int i = someFunc(); i >= 0) > { > // use i > } > Thoughts on this sort of feature? Personally, I'd love to have it, and I think that it's been suggested before (though I don't remember how that discussion went). I think that it would be a nice syntactic improvement over what Stefan suggested with an additional set of braces. It also would work with the else if case, whereas adding an extra scope would not. And I think that any chance of it actually being added to the language would hinge on its ability to either make stuff possible which is not currently possible or on its ability to make stuff that's really awkward easy to do in a non-awkward way. And there's a better case to be made that if(cond) { } else if(auto result = foo(); cond2) { } else if(cond3) { } is harder to emulate with the current language than doing it with the first if - though even with the first if branch, if you have something like if(auto result = foo(); cond1) { } else if(cond2) { } else if(cond3) { } the variable created in the first if branch would presumably leave scope immediately after the condition failed, which would also be much more difficult to emulate by adding additional braces. So, maybe it could be argued successfully with a DIP, but in general at this point, I think that Walter and Andrei are very resistant to making changes to the language - especially when it's simply for aesthetic benefit. So, any proposing a feature needs to be able to show that it brings real value. And while I, personally, think that this brings real value for certain use cases, I don't know if it brings enough value in general to be worth the addition, and more importantly, I don't know if Walter or Andrei would think that it's worth it, and they're the ones who need to be convinced. Ultimately though, the biggest hurdle is that someone needs to create a DIP for it that strongly argues its case with real world examples of how it would improve code (preferably with code from Phobos and code.dlang.org), and without a really well-written DIP it's going to be dead in the water unless Walter or Andrei already happened to have wished for something like that often enough that they didn't take much convincing (which I wouldn't bet on). Right now, the bar to get a new feature in is pretty high even simply from the standpoint of the amount of work required to successfully propose it. - Jonathan M Davis
Re: If Statement with Declaration
On Friday, 4 November 2016 at 19:26:23 UTC, Steven Schveighoffer wrote: I think it makes it easier to read and it fits with how the for-statement operates. I write code everyday that could utilize this if-statement syntax, so I thought I might as well bring it up. But if there isn't that much interest in it then I won't bother with a DIP. Please bear in mind that I'm not the gatekeeper, so what I say may not be what the actual ones in control think. It's possible that Walter and Andrei like the idea and would implement if someone fleshed out the proposal. In my experience, I have not encountered too many cases (definitely not zero though) where I needed such a feature. I can see the utility, and I wouldn't be opposed to it. -Steve No but you do make a compelling case. Having the scope structured the way you did removed the extra indention and it isn't as error prone as the other methods. You could also have multiple declarations which I don't think work (at least in the C++ implementation) for the new if-statement. {int a; double b; if(func(, ) >= 0) { }}
Re: If Statement with Declaration
On Friday, 4 November 2016 at 18:05:51 UTC, Adrian Matoga wrote: On Friday, 4 November 2016 at 15:34:00 UTC, Jerry wrote: On Friday, 4 November 2016 at 14:16:44 UTC, Matthias Bentrup wrote: On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry wrote: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I would prefer the syntax if( (int i = someFunc()) >= 0 ) { // use i } as this matches the already existing assignment expression syntax if you pull the declaration out of the if expression. Well you can argue the "if( init ; condition )" syntax matches "for( init ; condition ; update)", minus the "update" section. You've just answered your own question: for (int i = someFunc(); i >= 0; ) { // use i break; } Yes, it was one of the examples I gave in the first post. Reason not to use that is, if you forget "break" you are probably going to have an infinite loop. As well you can't use "else" with a for statement.
Re: If Statement with Declaration
On 04.11.2016 21:03, Andrej Mitrovic wrote: On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). This is already possible in library code. See here: https://github.com/AndrejMitrovic/minilib/blob/510460ff1381f765a66aa3b8f8f6d7e95b4597b9/src/minilib/core/types.d#L88-L137 Not worth the IFTI-induced template bloat IMHO.
Re: If Statement with Declaration
On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). This is already possible in library code. See here: https://github.com/AndrejMitrovic/minilib/blob/510460ff1381f765a66aa3b8f8f6d7e95b4597b9/src/minilib/core/types.d#L88-L137 One could come up with a helper function that encodes the condition which is moved into the opCast method. This should be trivial to implement.
Re: If Statement with Declaration
On 11/4/16 1:46 PM, Jerry wrote: On Friday, 4 November 2016 at 16:15:21 UTC, Steven Schveighoffer wrote: It's just a strawman type, I'm sure there's ways to handle these things, I just didn't put a lot of effort into all the cases. But really it's just syntax to separate the bool check from the value itself. You can get more elaborate if you want with the comparison. The difficult thing is having a type that you use normally, but that when you use in an if statement, it gives what you want. And declaring that type in the if clause itself (which is allowed in restricted circumstances). This also isn't exactly ideal for things like int, as if(i) would now be unexpected. It is just a basic type but ultimately you will probably need more than one type with a different way of operating to get the same behavior. That and having part of the code as a string which makes it harder to read and no syntax highlighting for the code in text editors/ides. What do you mean if(i) would be unexpected? I mean something like: if(auto i = func.cond("==0")) { assert(i == 0); if(i) { writeln("oops!"); } } Sure, it doesn't look great. But my expectation is that your proposal doesn't pull enough weight to be integrated into the language. There are enough ways to solve this problem that don't involve language changes. It follows the same rules for an if statement condition which an if statement allows "if(auto i = foo())". Right, I'm just saying syntax changes like this generally need a high motivation and perceived improvement to justify the undertaking. For instance, we have trusted escapes via this construct: auto foo = (() @trusted => systemFunc(x))(); It's ugly, verbose, confusing. But it works, and it works today. I'd much rather have this look like: auto foo = @trusted(systemFunc(x)); or something similar, but how much benefit are we going to get from this? Is it going to change productivity? Is it going to make code so much easier to write that the time taken to implement was worth it? I don't know the answer to that. Often the answer to requests for such improvements is "yeah, but you can just do it this way, and that's good enough". That's all I'm trying to say. It's syntactic sugar, most of the existing language features don't serve a functional purpose. They simply serve to simplify another syntax. Every "foreach" statement could be written as a for-statement. Every for-statement could be written as a while-statement. And so on so forth. It doesn't add anything that new either. It follows the same syntax as a for-statement. Existence of current sugar is not justification of adding more. Those already exist (and BTW have existed for a long long time), there is no cost to continuing to have them. There is a cost to adding new features, that we must weigh against the benefits. One thing that could work in your favor is if the change is easy to implement in the compiler. That I definitely don't know the answer to. I think it makes it easier to read and it fits with how the for-statement operates. I write code everyday that could utilize this if-statement syntax, so I thought I might as well bring it up. But if there isn't that much interest in it then I won't bother with a DIP. Please bear in mind that I'm not the gatekeeper, so what I say may not be what the actual ones in control think. It's possible that Walter and Andrei like the idea and would implement if someone fleshed out the proposal. In my experience, I have not encountered too many cases (definitely not zero though) where I needed such a feature. I can see the utility, and I wouldn't be opposed to it. -Steve
Re: If Statement with Declaration
On Friday, 4 November 2016 at 15:34:00 UTC, Jerry wrote: On Friday, 4 November 2016 at 14:16:44 UTC, Matthias Bentrup wrote: On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry wrote: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I would prefer the syntax if( (int i = someFunc()) >= 0 ) { // use i } as this matches the already existing assignment expression syntax if you pull the declaration out of the if expression. Well you can argue the "if( init ; condition )" syntax matches "for( init ; condition ; update)", minus the "update" section. You've just answered your own question: for (int i = someFunc(); i >= 0; ) { // use i break; }
Re: If Statement with Declaration
On Friday, 4 November 2016 at 16:15:21 UTC, Steven Schveighoffer wrote: It's just a strawman type, I'm sure there's ways to handle these things, I just didn't put a lot of effort into all the cases. But really it's just syntax to separate the bool check from the value itself. You can get more elaborate if you want with the comparison. The difficult thing is having a type that you use normally, but that when you use in an if statement, it gives what you want. And declaring that type in the if clause itself (which is allowed in restricted circumstances). This also isn't exactly ideal for things like int, as if(i) would now be unexpected. It is just a basic type but ultimately you will probably need more than one type with a different way of operating to get the same behavior. That and having part of the code as a string which makes it harder to read and no syntax highlighting for the code in text editors/ides. What do you mean if(i) would be unexpected? I thought you could only declare variables in the first clause? In any case, I think Stefan really has the best answer: {int value; if(auto i = someFunc()) { // ... }} Sure, it doesn't look great. But my expectation is that your proposal doesn't pull enough weight to be integrated into the language. There are enough ways to solve this problem that don't involve language changes. -Steve It follows the same rules for an if statement condition which an if statement allows "if(auto i = foo())". It's syntactic sugar, most of the existing language features don't serve a functional purpose. They simply serve to simplify another syntax. Every "foreach" statement could be written as a for-statement. Every for-statement could be written as a while-statement. And so on so forth. It doesn't add anything that new either. It follows the same syntax as a for-statement. It's not that big of a language feature, it's just a small syntactic change, which comes down to. Is this easier to read compared to this. {int value; if(auto i = someFunc()) { // ... }} vs. if(int value; auto i = someFunc()) { // ... } I think it makes it easier to read and it fits with how the for-statement operates. I write code everyday that could utilize this if-statement syntax, so I thought I might as well bring it up. But if there isn't that much interest in it then I won't bother with a DIP.
Re: If Statement with Declaration
On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry wrote: Thoughts on this sort of feature? Love your name, Jerry ;) Maybe something like this psuedo code: struct Condition(T) { Condition(T val, bool cond) { m_val = val; m_cond = cond; } bool opCast(T)() if(is(T == bool)) { return m_cond; } alias m_val this; private: T m_val; bool m_cond; } auto cond(T v, bool cond) { return Condition!T(v, cond); } Usage: int v = 5; if(v = cond(v, v > 4) { writeln("Happy coding"); } Just notice I am not proud of that T.init.
Re: If Statement with Declaration
On Friday, 4 November 2016 at 13:56:57 UTC, Andrea Fontana wrote: If you don't like indentation you can simply ignore it or you can use old goto :) { int i = someFunc(); if (i < 0) goto outer; // your code here } outer: BTW there is a trick to avoid goto+label: switch (true) { default: int i = someFunc(); if (i < 0) break; ... } If code after goto is long, the reader has to search for the label. With switch/break, the reader knows the current scope is just skipped, no need to interrupt reading of lines. We could allow omitting the condition and default label: switch { int i = someFunc(); if (i < 0) break; ... } That would encourage the pattern to be used instead of the 'triangle if' pattern - the rationale for breakable blocks: https://issues.dlang.org/show_bug.cgi?id=8622
Re: If Statement with Declaration
On 11/4/16 11:27 AM, Jerry wrote: On Friday, 4 November 2016 at 14:10:51 UTC, Steven Schveighoffer wrote: Hm... what about something like: struct BoolCond(T, string cond) { T val; bool opCast(B)() if(is(B == bool)) { return mixin("val " ~ cond); } } auto boolcond(string cond, T)(T val) { return BoolCond!(T, cond)(val); } if(auto i = someFunc.boolcond!(">= 0")) { ... // use i } Well that's just a basic case, what if you want more than one condition. Using "&&" for example, or a condition along with another value that's in the scope. It's just a strawman type, I'm sure there's ways to handle these things, I just didn't put a lot of effort into all the cases. But really it's just syntax to separate the bool check from the value itself. You can get more elaborate if you want with the comparison. The difficult thing is having a type that you use normally, but that when you use in an if statement, it gives what you want. And declaring that type in the if clause itself (which is allowed in restricted circumstances). This also isn't exactly ideal for things like int, as if(i) would now be unexpected. Then you need another work around for something like this: if(int value; auto i = someFunc()) { // ... } I thought you could only declare variables in the first clause? In any case, I think Stefan really has the best answer: {int value; if(auto i = someFunc()) { // ... }} Sure, it doesn't look great. But my expectation is that your proposal doesn't pull enough weight to be integrated into the language. There are enough ways to solve this problem that don't involve language changes. -Steve
Re: If Statement with Declaration
On Friday, 4 November 2016 at 14:10:51 UTC, Steven Schveighoffer wrote: On 11/3/16 6:29 PM, Jerry wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? Hm... what about something like: struct BoolCond(T, string cond) { T val; bool opCast(B)() if(is(B == bool)) { return mixin("val " ~ cond); } } auto boolcond(string cond, T)(T val) { return BoolCond!(T, cond)(val); } if(auto i = someFunc.boolcond!(">= 0")) { ... // use i } -Steve Well that's just a basic case, what if you want more than one condition. Using "&&" for example, or a condition along with another value that's in the scope. void otherFunc(int input) { if(auto i = someFunc.boolcond!(">= input")) // --- error { ... // use i } } Then you need another work around for something like this: if(int value; auto i = someFunc()) { // ... }
Re: If Statement with Declaration
On Friday, 4 November 2016 at 14:16:44 UTC, Matthias Bentrup wrote: On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry wrote: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I would prefer the syntax if( (int i = someFunc()) >= 0 ) { // use i } as this matches the already existing assignment expression syntax if you pull the declaration out of the if expression. Well you can argue the "if( init ; condition )" syntax matches "for( init ; condition ; update)", minus the "update" section. It's cleaner as well as you don't need an extra set of parenthesizes. As well that sort of syntax would allow multiple declaration. Which I'm not sure that's desired, it becomes increasingly messy. if((int i = someFunc()) >= 0 && (int j = otherFunc()) >= 0) { } You also can't use the declaration as input for someFunc(): if(int i; someFunc() >= 0) { }
Re: If Statement with Declaration
On 11/4/16 10:10 AM, Steven Schveighoffer wrote: On 11/3/16 6:29 PM, Jerry wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? Hm... what about something like: struct BoolCond(T, string cond) { T val; bool opCast(B)() if(is(B == bool)) { return mixin("val " ~ cond); } Of course, I missed this: alias val this; -Steve
Re: If Statement with Declaration
On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry wrote: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? I would prefer the syntax if( (int i = someFunc()) >= 0 ) { // use i } as this matches the already existing assignment expression syntax if you pull the declaration out of the if expression.
Re: If Statement with Declaration
On 11/3/16 6:29 PM, Jerry wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature? Hm... what about something like: struct BoolCond(T, string cond) { T val; bool opCast(B)() if(is(B == bool)) { return mixin("val " ~ cond); } } auto boolcond(string cond, T)(T val) { return BoolCond!(T, cond)(val); } if(auto i = someFunc.boolcond!(">= 0")) { ... // use i } -Steve
Re: If Statement with Declaration
On Friday, 4 November 2016 at 13:38:30 UTC, Superstar64 wrote: On Thursday, 3 November 2016 at 22:32:17 UTC, Stefan Koch wrote: Just Introduce another block { int i = someFunc(); if (i >= 0) { ... } } // i is not visible here That adds 2 indention levels after formatting. Unfortunately this doesn't work: --- { int i = someFunc(); if (i >= 0): //your code here } // i is not visible here --- If you don't like indentation you can simply ignore it or you can use old goto :) { int i = someFunc(); if (i < 0) goto outer; // your code here } outer: // i is not visibe here
Re: If Statement with Declaration
On Thursday, 3 November 2016 at 22:32:17 UTC, Stefan Koch wrote: Just Introduce another block { int i = someFunc(); if (i >= 0) { ... } } // i is not visible here That adds 2 indention levels after formatting. Unfortunately this doesn't work: --- { int i = someFunc(); if (i >= 0): //your code here } // i is not visible here ---
Re: If Statement with Declaration
On Friday, 4 November 2016 at 00:04:28 UTC, mogu wrote: Introducing a block is not intuitive and cause an identation. How is using a block to produce a sub-scope not intuitive? That's using a standard feature to do exactly what it is supposed to do.
Re: If Statement with Declaration
On Thursday, 3 November 2016 at 22:32:17 UTC, Stefan Koch wrote: On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: [...] Just Introduce another block { int i = someFunc(); if (i >= 0) { ... } } // i is not visible here Introducing a block is not intuitive and cause an identation. As a reference, C++17 has adopted a proposal about this, and extended to switch: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0305r1.html
Re: If Statement with Declaration
On Thursday, 3 November 2016 at 22:29:34 UTC, Jerry wrote: So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: [...] Just Introduce another block { int i = someFunc(); if (i >= 0) { ... } } // i is not visible here
If Statement with Declaration
So I was thinking of a way of extending if statements that have declarations. The following being as example of the current use of if statements with declarations: if(int* weDontPollute = someFunc()) { // use weDontPollute } That's great and all, but it only works by checking if the variable evaluates to true or false. Which is fine for a pointer but otherwise useless for anything else, like integers where zero is usually valid input (index for array). So currently the only way to do something like this in the language, that i've found, is to use a for statement. for(int i = someFunc(); i >= 0;) { // use i break; } Not that ideal to use a for statement. It makes it hard to read and if the break condition isn't there it might very well be an infinite loop. So I was thinking of some sort of syntax like this: if(int i = someFunc(); i >= 0) { // use i } Thoughts on this sort of feature?