[Issue 6662] New: std.functional.memoize with structs

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=6662

           Summary: std.functional.memoize with structs
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: rejects-valid
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2011-09-13 04:18:51 PDT ---
In the following D2 program only the Mfoo1 compiles:


import std.functional: memoize;
struct Foo1 {
    int x;
    static Foo1 opCall(int x_) {
        Foo1 f;
        f.x = x_;
        return f;
    }
}
struct Foo2 {
    int x;
}
struct Foo3 {
    int x;
    this(int x_) {
        this.x = x_;
    }
}
void main() {
    alias memoize!Foo1 Mfoo1;
    alias memoize!Foo2 Mfoo2;
    alias memoize!Foo3 Mfoo3;
}


DMD 2.055 gives errors like:

...\src\phobos\std\traits.d(128): Error: static assert  "argument has no return
type"
...\src\phobos\std\functional.d(713):        instantiated from here:
ReturnType!(Foo2)
test.d(21):        instantiated from here: memoize!(Foo2)

...\src\phobos\std\traits.d(128): Error: static assert  "argument has no return
type"
...\src\phobos\std\functional.d(713):        instantiated from here:
ReturnType!(Foo3)
test.d(22):        instantiated from here: memoize!(Foo3)


The static assert is in std.traits:

template ReturnType(/+@@@BUG4217@@@+/func...)
    if (/+@@@BUG4333@@@+/staticLength!(func) == 1)
{
    static if (is(FunctionTypeOf!(func) R == return))
        alias R ReturnType;
    else
        static assert(0, "argument has no return type");
}


I think Mfoo2 and Mfoo3 too should compile. Writing Foo2(5) is like calling the
function Foo2 with argument x = 5, and returning a struct Foo2 result. I think
this is a place where telling apart structs and functions is an artificial
distinction that is the opposite of useful.


See one use case:
http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=144333

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