std.functional.curry isn't

[Philippe Sigaud]

On Fri, Jun 25, 2010 at 16:36, Andrei Alexandrescu <
SeeWebsiteForEmail at erdani.org> wrote:

> On 06/25/2010 08:39 AM, Graham Fawcett wrote:
>
>> Thank you.
>>
>> http://d.puremagic.com/issues/show_bug.cgi?id=4391
>>
>
> Great. I'm hoping for a patch that fixes the problem, but it's good to have
> the report we don't forget this.


I'm using this, without problem up to now, it works for n-args functions
(even 0 or 1) and accepts template functions too, but see the doc for the
limitation in this case.


/**
Takes a D function, and curries it (in traditional, mathematical sense):
given
a n-args function, it creates n 1-arg functions nested inside one another.
When
all original arguments are reached, it returns the result.

Example:
----
int add(int i, int j) { return i+j;}
alias curry!add cadd; // cadd waits for an int, will return an int
delegate(int)
auto add3 = cadd(3); // add3 is a function that take an int and return this
int + 3.

auto m = map!add3([0,1,2,3]);
assert(equal(m, [3,4,5,6]));

bool equals(int i, int j) { return i==j;}
alias curry!equals cequals;
auto equals4 = cequals(4); // equals4 is a function taking an int and return
true iff this int is 4.
auto f = filter!equals4([2,3,4,5,4,3,2,2,3,4]);
assert(equal(f, [4,4,4]));
----

What's fun is that it'll work for template functions too.

Example:
----
string conj(A, B)(A a, B b)
{
    return to!string(a)~to!string(b);
}

alias curry!conj cconj;
auto c1 = cconj(1); // c1 is a template function waiting for any type.
assert(c1('a') == "1a");
----
BUG:
for now, it does not verify the compatibility of types while you give it the
arguments. It's only
at the end that it sees whether or not it can call the function with these
arguments.
Example:
----
// given a function like this, with dependencies between the arguments'
types:
A foo(A,B,C)(A a, Tuple!(B,A) b, Tuple!(C,B,A) c) { return
a+b.field[1]+c.field[2];}
// if you curries it and gives it an int as first argument, the returned
template function should really be:
int foo2(B,C)(Tuple!(B,int) b) { return anotherFunction;}
// because we now know A to be an int...
----
*/
template curry(alias fun)
{
    static if (isFunction!fun)
        enum curry =  mixin(curryImpl!(fun, "", ParameterTypeTuple!fun));
    else
        enum curry = curriedFunction!(fun)();
}

template curryImpl(alias fun, string xs, T...)
{
    static if (T.length == 0)
        enum curryImpl = "&fun";
    else static if (T.length == 1)
        enum curryImpl = "(" ~ T[0].stringof  ~ " x1) { return fun(" ~ xs ~
"x1);}";
    else
        enum curryImpl = "(" ~ T[0].stringof  ~ " x" ~ to!string(T.length) ~
") { return "
                            ~ curryImpl!(fun,xs ~ "x" ~ to!string(T.length)
~ ", ", T[1..$]) ~ ";}";
}

struct CurriedFunction(alias fun, T...) /+if (T.length)+/
{
    T _t;
    static if (T.length)
        void initialize(T t) { _t = t;}

    template OpCallType(U...)
    {
        static if (is (typeof(fun(Init!T, Init!U))))
            alias typeof(fun(Init!T, Init!U)) OpCallType;
        else
            alias CurriedFunction!(fun, T, U) OpCallType;
    }

    OpCallType!U opCall(U...)(U u)
    {
        static if(is(typeof(fun(_t, u))))
            return fun(_t,u);
        else
        {
            CurriedFunction!(fun, T, U) cf;
            static if (U.length) cf.initialize(_t, u);
            return cf;
        }
    }
}

CurriedFunction!(fun, TypeTuple!()) curriedFunction(alias fun)()
{
    CurriedFunction!(fun, TypeTuple!()) cf;
    return cf;
}

unittest
{
    int add(int i, int j) { return i+j;}
    alias curry!add cadd; // cadd waits for an int, will return an int
delegate(int)
    auto add3 = cadd(3); // add3 is a function that take an int and return
this int + 3.

    auto m = map!add3([0,1,2,3]);
    assert(equal(m, [3,4,5,6]));

    bool equals(int i, int j) { return i==j;}
    alias curry!equals cequals;
    auto equals4 = cequals(4); // equals4 is a function taking an int and
return true iff this int is 4.
    auto f = filter!equals4([2,3,4,5,4,3,2,2,3,4]);
    assert(equal(f, [4,4,4]));
}


So, the template function part is still a bit rough. I've plan to keep track
of types dependencies, but ... well, Real Life (tm) is in the way.

I posted it as a comment in the bug report, in case it can help someone
write something.

Philippe
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