Re: "else if" for template constraints
On Friday, 4 September 2015 at 15:52:08 UTC, Enamex wrote:
The biggest problem, I think, is that a template can has
multiple 'predicates' to agree to be instantiated, but only
some of them can mutually exclusive (the specialization syntax
produces mutually exclusive ones, the if-constraints don't).
[...]
template Bar(T) {
static if( is(T == int) || is(T == string) || ... ) {
//stuff
}
else static if( stuff ) {
// other stuff
}
else {
template return; // I know my T took whatever your type
was but I actually don't match, please exclude me from your
list for this instance...
}
}
[...]
On second thought, it wouldn't help as much as I'd thought with
overloading problems. What we want is a 'which template has more
'&&'ed expressions in its constraint?' which sounds pretty awful.
I have no idea how this could work :/
Especially given that D's constraints are way more open than,
say, Haskell's, in their checking; though ironically not their
declaration/implementation (D is open because it checks for
structural conformance of a struct instead of nominative &
structural; but it's more restricted because it easily only
checks for structure of the type as declared, with no way to
attach recognized-to-the-constraint ad-hoc interfaces to a type
like Haskell's with type-classes).
Re: "else if" for template constraints
On Monday, 17 August 2015 at 13:18:43 UTC, Steven Schveighoffer
wrote:
I was just looking at fixing this
bug:https://issues.dlang.org/show_bug.cgi?id=14925
[...]
How often are you writing overloaded templates, and you want to
say "if it doesn't match anything else, do this"? I'd love to
see some form of syntax that brings template constraints in
line with tried-and-true if/else statements.
One way to do this is to lexically order the if constraints,
and if any of them start with "else", then they are mutually
exclusive with the immediately preceding constraint for the
same symbol (just like normal else).
So for example, you'd have:
void replaceInPlace(T, Range)(ref T[] array, size_t from,
size_t to, Range stuff)
if(isDynamicArray!Range &&
is(Unqual!(ElementEncodingType!Range) == T) &&
!is(T == const T) &&
!is(T == immutable T))
{ /* version 1 that tries to write into the array directly */ }
void replaceInPlace(T, Range)(ref T[] array, size_t from,
size_t to,
Range stuff)
else if(is(typeof(replace(array, from, to, stuff
{ /* version 2, which simply forwards to replace */ }
looks much better IMO. Can we do something like this? I'm not a
compiler guru, so I defer to you experts out there.
-Steve
The biggest problem, I think, is that a template can has multiple
'predicates' to agree to be instantiated, but only some of them
can mutually exclusive (the specialization syntax produces
mutually exclusive ones, the if-constraints don't).
Thinking about it from this angle, I believe the most flexible
and sensible solution would be to support a sort of "early
return" from a template. Thus:
template Bar(T) {
static if( is(T == int) || is(T == string) || ... ) {
//stuff
}
else static if( stuff ) {
// other stuff
}
else {
template return; // I know my T took whatever your type
was but I actually don't match, please exclude me from your list
for this instance...
}
}
template Bar(T) {
static if( is(T == float) || is(T == int[]) || ... ) {
// Bar!float/Bar!(int[]) stuff
}
else static if( OTHER_OTHER_stuff ) {
// other other stuff
}
else {
template return; // I know my T took whatever your type
was but I actually don't match, please exclude me from your list
for this instance...
}
}
Re: else if for template constraints
On 8/17/15 6:44 PM, anonymous wrote:
On Monday, 17 August 2015 at 22:32:10 UTC, Idan Arye wrote:
On Monday, 17 August 2015 at 21:27:47 UTC, Meta wrote:
[...]
At that point, couldn't you just use static if inside the body of the
template instead of using template constraints?
No. Consider this: http://dpaste.dzfl.pl/a014aeba6e68. The having two
foo templates is illegal(though it'll only show when you try to
instantiate foo), because each of them covers all options for T. When
T is neither int nor float, the foo *function* in the first template
is not defined, but the *foo* template is still there.
The idea is to have only one template:
template foo(T) {
static if (is(T == int)) {
...
} else static if (is(T == float)) {
...
} else static if (is(T == char)) {
...
} else static if (is(T == bool)) {
...
}
}
What if there is another foo template that handles double? possibly in
another file? There would be a conflict for instantiation.
That is why we use template constraints instead of static if -- the
constraint disqualifies the instantiation.
-Steve
Re: else if for template constraints
On Monday, 17 August 2015 at 21:27:47 UTC, Meta wrote:
On Monday, 17 August 2015 at 17:17:15 UTC, Steven Schveighoffer
wrote:
On 8/17/15 1:00 PM, Idan Arye wrote:
It looks a bit ugly, that the `else` is after a function
declaration
instead of directly after the if's then clause. How about
doing it
with the full template style?
template replaceInPlace(T, Range)
if(isDynamicArray!Range
is(Unqual!(ElementEncodingType!Range) == T)
!is(T == const T)
!is(T == immutable T))
{
void replaceInPlace(ref T[] array, size_t from,
size_t to,
Range stuff)
{ /* version 1 that tries to write into the array
directly */ }
}
else if(is(typeof(replace(array, from, to, stuff
{
void replaceInPlace(ref T[] array, size_t from,
size_t to,
Range stuff)
{ /* version 2, which simply forwards to replace */ }
}
Yes, I like this much better.
-Steve
At that point, couldn't you just use static if inside the body
of the template instead of using template constraints?
No. Consider this: http://dpaste.dzfl.pl/a014aeba6e68. The having
two foo templates is illegal(though it'll only show when you try
to instantiate foo), because each of them covers all options for
T. When T is neither int nor float, the foo *function* in the
first template is not defined, but the *foo* template is still
there.
With the suggested syntax, the first foo template would only be
defined for int and float, and the second will only be defined
for char and bool - so there is no conflict.
Re: else if for template constraints
On Monday, 17 August 2015 at 22:32:10 UTC, Idan Arye wrote:
On Monday, 17 August 2015 at 21:27:47 UTC, Meta wrote:
[...]
At that point, couldn't you just use static if inside the body
of the template instead of using template constraints?
No. Consider this: http://dpaste.dzfl.pl/a014aeba6e68. The
having two foo templates is illegal(though it'll only show when
you try to instantiate foo), because each of them covers all
options for T. When T is neither int nor float, the foo
*function* in the first template is not defined, but the *foo*
template is still there.
The idea is to have only one template:
template foo(T) {
static if (is(T == int)) {
...
} else static if (is(T == float)) {
...
} else static if (is(T == char)) {
...
} else static if (is(T == bool)) {
...
}
}
Re: else if for template constraints
On Monday, 17 August 2015 at 22:44:15 UTC, anonymous wrote:
On Monday, 17 August 2015 at 22:32:10 UTC, Idan Arye wrote:
On Monday, 17 August 2015 at 21:27:47 UTC, Meta wrote:
[...]
At that point, couldn't you just use static if inside the
body of the template instead of using template constraints?
No. Consider this: http://dpaste.dzfl.pl/a014aeba6e68. The
having two foo templates is illegal(though it'll only show
when you try to instantiate foo), because each of them covers
all options for T. When T is neither int nor float, the foo
*function* in the first template is not defined, but the *foo*
template is still there.
The idea is to have only one template:
template foo(T) {
static if (is(T == int)) {
...
} else static if (is(T == float)) {
...
} else static if (is(T == char)) {
...
} else static if (is(T == bool)) {
...
}
}
There is also, as a similar option, the dispatcher solution,
like for 'std.conv.to'
There is the main template that dispatches the call to the right
non templated (or specialized) overload, so that the entry point
is just used to redirect the cases that are fundamentaly
different.
---
auto foo(T)
{
static if(...) return fooImpl!T(); else
static if(...) return fooImpl!T(); else
//etc
}
private auto fooImpl(T)(){}
private auto fooImpl(T)(){}
private auto fooImpl(T)(){}
---
Re: else if for template constraints
On 8/17/15 9:18 AM, Steven Schveighoffer wrote: The issue (as I noted in the bug report), is that the array being replaced is some string, and the element type of the stuff to replace is a dchar. But the first version is better for replacing a char[] in a char[], and works just fine. I guess this was a slightly wrong opinion. Because the first overload uses remove, and remove cannot work with char[] (see phobos schizophrenia regarding strings), it must call the always-allocating second version. In any case, the template constraints are still simpler with the new style (if(!priorconstraints) ...) -Steve
else if for template constraints
I was just looking at fixing this
bug:https://issues.dlang.org/show_bug.cgi?id=14925
A little background for the root cause:
replaceInPlace has 2 versions. One is a specialized version that
replaces the actual elements in an array with another array of the same
type.
The second version just uses replace, and then overwrites the original
array reference. This is used when the stuff to replace is not an array,
or the array elements don't match, or the target array has const or
immutable elements.
The constraint for version 1 is:
if(isDynamicArray!Range
is(ElementEncodingType!Range : T)
!is(T == const T)
!is(T == immutable T))
More on that later. The constraint for version 2 is:
if(isInputRange!Range
((!isDynamicArray!Range is(ElementType!Range : T)) ||
(isDynamicArray!Range is(ElementType!Range : T)
(is(T == const T) || is(T == immutable T))) ||
isSomeString!(T[]) is(ElementType!Range : dchar)))
The issue (as I noted in the bug report), is that the array being
replaced is some string, and the element type of the stuff to replace
is a dchar. But the first version is better for replacing a char[] in a
char[], and works just fine.
So I set about fixing this third constraint. We need to only call this
version if the some string actually has non-mutable characters. So I
proceeded to add another is(T == const T) || is(T == immutable T), but
then I realized, wait, the first version will also be called if the
ElementEncodingType of Range *fits into* a T (note the use of a colon
instead of ==). Therefore, replaceInPlace!(wchar, char[]) will
incorrectly try and call the first version, when it should call the second.
Therefore, I needed to update the constraints on the first version. This
screws up the whole dynamic, because both constraints are evaluated
INDEPENDENTLY. You can't have any overlap, so any changes to the first
constraint may cause issues with the second (and in this case, it does).
My second set of constraints was starting to look REALLY complicated.
What I really need for the second constraint is: doesn't match the
first version AND I can call replace with those arguments.
So that's what I did:
if(!(... /* whole constraint from first version */)
is(typeof(replace(array, from, to, stuff
This is not very DRY. One thing I could do is factor out the main
constraint into another template:
enum _replaceInPlaceConstraint(T, Range) = ...
Great! But then the docs don't reflect the true constraint (they just
have this _replaceInPlaceConstraint, er.. constraint), and I've
contributed to the ever-growing template bloat of phobos.
How often are you writing overloaded templates, and you want to say if
it doesn't match anything else, do this? I'd love to see some form of
syntax that brings template constraints in line with tried-and-true
if/else statements.
One way to do this is to lexically order the if constraints, and if any
of them start with else, then they are mutually exclusive with the
immediately preceding constraint for the same symbol (just like normal
else).
So for example, you'd have:
void replaceInPlace(T, Range)(ref T[] array, size_t from, size_t to,
Range stuff)
if(isDynamicArray!Range
is(Unqual!(ElementEncodingType!Range) == T)
!is(T == const T)
!is(T == immutable T))
{ /* version 1 that tries to write into the array directly */ }
void replaceInPlace(T, Range)(ref T[] array, size_t from, size_t to,
Range stuff)
else if(is(typeof(replace(array, from, to, stuff
{ /* version 2, which simply forwards to replace */ }
looks much better IMO. Can we do something like this? I'm not a compiler
guru, so I defer to you experts out there.
-Steve
Re: else if for template constraints
On 08/17/2015 03:18 PM, Steven Schveighoffer wrote: Can we do something like this? I'm not a compiler guru, so I defer to you experts out there. Implementation is trivial. (A naive implementation strategy which works is to just use the obvious lowering.)
Re: else if for template constraints
On Monday, 17 August 2015 at 13:18:43 UTC, Steven Schveighoffer
wrote:
void replaceInPlace(T, Range)(ref T[] array, size_t from,
size_t to, Range stuff)
if(isDynamicArray!Range
is(Unqual!(ElementEncodingType!Range) == T)
!is(T == const T)
!is(T == immutable T))
{ /* version 1 that tries to write into the array directly */ }
void replaceInPlace(T, Range)(ref T[] array, size_t from,
size_t to,
Range stuff)
else if(is(typeof(replace(array, from, to, stuff
{ /* version 2, which simply forwards to replace */ }
It looks a bit ugly, that the `else` is after a function
declaration instead of directly after the if's then clause. How
about doing it with the full template style?
template replaceInPlace(T, Range)
if(isDynamicArray!Range
is(Unqual!(ElementEncodingType!Range) == T)
!is(T == const T)
!is(T == immutable T))
{
void replaceInPlace(ref T[] array, size_t from, size_t
to, Range stuff)
{ /* version 1 that tries to write into the array
directly */ }
}
else if(is(typeof(replace(array, from, to, stuff
{
void replaceInPlace(ref T[] array, size_t from, size_t
to, Range stuff)
{ /* version 2, which simply forwards to replace */ }
}
Re: else if for template constraints
On Monday, 17 August 2015 at 16:57:33 UTC, Zoadian wrote: wouldn't is(typeof(replace(array, from, to, stuff))) better be a static if inside the first version? nevermind, I missed that the first constraint is negated. In that case I agree, else if would be nice.
Re: else if for template constraints
On Monday, 17 August 2015 at 13:18:43 UTC, Steven Schveighoffer
wrote:
void replaceInPlace(T, Range)(ref T[] array, size_t from,
size_t to, Range stuff)
if(isDynamicArray!Range
is(Unqual!(ElementEncodingType!Range) == T)
!is(T == const T)
!is(T == immutable T))
{ /* version 1 that tries to write into the array directly */ }
void replaceInPlace(T, Range)(ref T[] array, size_t from,
size_t to,
Range stuff)
else if(is(typeof(replace(array, from, to, stuff
{ /* version 2, which simply forwards to replace */ }
looks much better IMO. Can we do something like this? I'm not a
compiler guru, so I defer to you experts out there.
-Steve
wouldn't is(typeof(replace(array, from, to, stuff))) better be a
static if inside the first version?
Re: else if for template constraints
On 8/17/15 1:00 PM, Idan Arye wrote:
It looks a bit ugly, that the `else` is after a function declaration
instead of directly after the if's then clause. How about doing it
with the full template style?
template replaceInPlace(T, Range)
if(isDynamicArray!Range
is(Unqual!(ElementEncodingType!Range) == T)
!is(T == const T)
!is(T == immutable T))
{
void replaceInPlace(ref T[] array, size_t from, size_t to,
Range stuff)
{ /* version 1 that tries to write into the array directly */ }
}
else if(is(typeof(replace(array, from, to, stuff
{
void replaceInPlace(ref T[] array, size_t from, size_t to,
Range stuff)
{ /* version 2, which simply forwards to replace */ }
}
Yes, I like this much better.
-Steve
Re: else if for template constraints
On Monday, 17 August 2015 at 17:17:15 UTC, Steven Schveighoffer
wrote:
On 8/17/15 1:00 PM, Idan Arye wrote:
It looks a bit ugly, that the `else` is after a function
declaration
instead of directly after the if's then clause. How about
doing it
with the full template style?
template replaceInPlace(T, Range)
if(isDynamicArray!Range
is(Unqual!(ElementEncodingType!Range) == T)
!is(T == const T)
!is(T == immutable T))
{
void replaceInPlace(ref T[] array, size_t from,
size_t to,
Range stuff)
{ /* version 1 that tries to write into the array
directly */ }
}
else if(is(typeof(replace(array, from, to, stuff
{
void replaceInPlace(ref T[] array, size_t from,
size_t to,
Range stuff)
{ /* version 2, which simply forwards to replace */ }
}
Yes, I like this much better.
-Steve
At that point, couldn't you just use static if inside the body of
the template instead of using template constraints?
