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?
