• d-bugmail puremagic.com (42/42) May 24 2012 http://d.puremagic.com/issues/show_bug.cgi?id=8142

d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=8142

           Summary: std.functional.curry isn't flexible enough
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc



std.functional.curry doesn't work if the first argument (the curried one) is a
function. This is a common enough use case:


import std.functional;
auto foo(T)(T function(T x) f, T x) {
    return f(x);
}
int bar(int x) { return x * 2; }
alias curry!(foo, &bar) fooBar;
void main() {}


DMD 2.060alpha gives:

temp.d(6): Error: expression & bar is not a valid template value argument
temp.d(6): Error: template instance std.functional.curry!(foo,& bar) error
instantiating

I think D functional-style programming needs a more flexible/uniform currying.


Workaround:

import std.traits: ParameterTypeTuple;
auto foo(alias F)(ParameterTypeTuple!F[0] x) {
    return F(x);
}
int bar(int x) { return x * 2; }
alias foo!bar fooBar;
void main() {
    assert(fooBar(5) == 10);
}


But this is not as flexible. A language that supports functional programming
needs to handle functions as first class citizens.

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