Auto returntype

[Alex]

On Monday, 20 May 2019 at 00:31:48 UTC, sarn wrote:
> On Sunday, 19 May 2019 at 23:10:08 UTC, Para wrote:
>> Unless I have very poor reading comprehension, which I might, 
>> I believe this is already a feature: 
>> https://dlang.org/spec/function.html#auto-functions
>
> The type of an auto function is inferred from the return 
> argument, but I think Alex wants more sophisticated inference 
> based on usage.  Maybe something like Hindley-Milner, as used 
> in Haskell: 
> http://www.codecommit.com/blog/scala/what-is-hindley-milner-and-why-is-it-cool
>

Yes, but it's much easier than that. No reason to infer anything. 
The type is the lvalue's type. but it requires one to determine 
that the lvalue has a type. In fact, maybe it could be a more 
general thing where one could simply get the lvalue(see below).

> D's templates could theoretically support more inference, but 
> that would need
> 1) Someone interested enough to implement it
> 2) Someone who can convince Walter that it won't make D's 
> template system too complicated

First it should be proved to be a sound idea, else it is 
pointless to do 1 and 2.

----


One could in general get the assignment type and use that in code 
to perform polymorphic behavior:


T foo()
{
    writeln(T.stringof);
    T x;
    return x;
}


int x = foo();
float x = foo();

prints

int
float


It requires that the lvalue's type be known and parsed before the 
template call... not a hard problem but might not be implemented 
in D as such.