Re: Doubt about type Inference on templates
On Wednesday, 22 November 2023 at 19:37:58 UTC, Paul Backus wrote:
This is a bug/limitation in the compiler. I couldn't find an
existing report on issues.dlang.org, so I've reported it myself
as [issue 24255][1].
Wow: It is a very concise bug example.
I tested with ```ldc``` ant it fails too.
For now, I think the best way to work around it is to specify
the type in the lambda, as in `(int i) => i%2 == 0`.
agreed
The reason you see `void` is that when the compiler cannot
figure out the type of a function literal, it treats it as a
template function:
```d
static assert(__traits(isTemplate, i => i % 2 == 0));
```
And for silly historical reasons, when the compiler tries to
determine the type of a template, it returns `void` instead of
giving an error:
```d
template example() {}
static assert(is(typeof(example) == void)); // what??
```
Thanks Paul!!!
Re: Doubt about type Inference on templates
On Wednesday, 22 November 2023 at 17:53:15 UTC, Antonio wrote:
Basically, it doesn't know witch version of ```filter``` to
use, because it is inferring `i=>i%2==0` is `void` ?!?!?!
```
!()(IIterable!int, void)
```
If I explicitly write `(int i)=>i%2==0`, it compiles correctly
again.
**Is it mandatory to explicitly tell that `S` is `int` when
```IIterable!S source``` is `IIterable!int` alredy?**
This is a bug/limitation in the compiler. I couldn't find an
existing report on issues.dlang.org, so I've reported it myself
as [issue 24255][1].
For now, I think the best way to work around it is to specify the
type in the lambda, as in `(int i) => i%2 == 0`.
The reason you see `void` is that when the compiler cannot figure
out the type of a function literal, it treats it as a template
function:
```d
static assert(__traits(isTemplate, i => i % 2 == 0));
```
And for silly historical reasons, when the compiler tries to
determine the type of a template, it returns `void` instead of
giving an error:
```d
template example() {}
static assert(is(typeof(example) == void)); // what??
```
[1]: https://issues.dlang.org/show_bug.cgi?id=24255
Doubt about type Inference on templates
Just for fun, I'm trying to implement an alternative base library
to avoid template/mixin/static/traits code with only one
objective: make "intelliSense" code analyzers tasks easier.
I need "Generics"... but D has not generics: I use templates in
the "simplest" possible way
I.E.:
```d
interface IIterable(T)
{
bool empty();
void popFront();
T front();
}
IIterable!S toIterable(S)(S[] source)
=> new ArrayIterable!S(source);
IIterable!S filter(S)(IIterable!S source, bool delegate(S item)
predicate)
=> new Filter!S(source, predicate);
IIterable!S filter(S)(S[] source, bool delegate(S item) predicate)
=> toIterable(source).filter(predicate);
// ...
```
Then, in main.d I do
```d
import std.stdio;
void main(){
[1,2,3,4,5,6].toIterable!int.filter!int(i=>i%2==0).map!int(i=>i*2).toArray.writeln();
}
```
It works properly... until I remove the ```!int``` from the
```filter``` method.
```
main.d(3,38): Error: none of the overloads of template `filter`
are callable using argument types `!()(IIterable!int, void)`
iterable.d(21,13):Candidates are: `filter(S)(IIterable!S
source, bool delegate(S item) predicate)`
iterable.d(23,13):`filter(S)(S[] source,
bool delegate(S item) predicate)`
```
Basically, it doesn't know witch version of ```filter``` to use,
because it is inferring `i=>i%2==0` is `void` ?!?!?!
```
!()(IIterable!int, void)
```
If I explicitly write `(int i)=>i%2==0`, it compiles correctly
again.
**Is it mandatory to explicitly tell that `S` is `int` when
```IIterable!S source``` is `IIterable!int` alredy?**
