Type Erasure In D

erasure.d

/*
Copyright (C) Freddy A Cubas "Superstar64" 2020

Boost Software License - Version 1.0 - August 17th, 2003

Permission is hereby granted, free of charge, to any person or organization
obtaining a copy of the software and accompanying documentation covered by
this license (the "Software") to use, reproduce, display, distribute,
execute, and transmit the Software, and to prepare derivative works of the
Software, and to permit third-parties to whom the Software is furnished to
do so, all subject to the following:

The copyright notices in the Software and this entire statement, including
the above license grant, this restriction and the following disclaimer,
must be included in all copies of the Software, in whole or in part, and
all derivative works of the Software, unless such copies or derivative
works are solely in the form of machine-executable object code generated by
a source language processor.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.
*/
import std.meta;
import std.traits : isPointer, PointerTarget;

struct SingletonKind(T)
{
    alias Representation = T;
    enum allows(K) = is(K == T);
}

struct PointerKind
{
    alias Representation = void*;
    enum allows(T : T*) = true;
    enum allows(T) = false;
}

struct TypeVariable(string name, Kind)
{
    Kind.Representation internal;
}

alias Primatives = AliasSeq!(void, bool, byte, ubyte, short, ushort, int, uint,
        long, ulong, float, double, real, ifloat, idouble, ireal, cfloat,
        cdouble, creal, char, wchar, dchar);

template SubstituteCurry(Context)
{
    alias SubstituteCurry(T) = Substitute!(T, Context);
}

template Substitute(T : TypeVariable!(name, Kind), Context, string name, Kind)
        if (Context.contains!name)
{
    static assert(Kind.allows!(Context.lookup!name));
    alias Substitute = Context.lookup!name;
}

template Substitute(T : TypeVariable!(name, Kind), Context, string name, Kind)
        if (!Context.contains!name)
{
    alias Substitute = T;
}

template Substitute(T, Context) if (staticIndexOf!(T, Primatives) != -1)
{
    alias Substitute = T;
}

template Substitute(T, Context) if (isPointer!T)
{
    alias Substitute = Substitute!(PointerTarget!T, Context)*;
}

alias Substitute(T : U[], Context, U) = Substitute!(U, Context)[];
alias Substitute(T : K[V], Context, K, V) = Substitute!(K, Context)[Substitute!(V, Context)];
alias Substitute(T : R function(A), Context, R, A...) = Substitute!(R, Context) function(
        staticMap!(SubstituteCurry!Context, A));
alias Substitute(T : R delegate(A), Context, R, A...) = Substitute!(R, Context) delegate(
        staticMap!(SubstituteCurry!Context, A));

auto specialize(string name, T, A)(A type)
{
    struct ThisContext
    {
        enum contains(string candidate) = name == candidate;
        alias lookup(string candidate : name) = T;
    }

    return *cast(Substitute!(A, ThisContext)*)(&type);
}

/*

translation of some haskell

id :: a -> a
id x = x

*/

TypeVariable!("a", PointerKind) identity(TypeVariable!("a", PointerKind) x)
{
    return x;
}

/*

curry :: ((a, b) -> c) -> a -> b -> c
curry f x y = f(x,y)

*/

TypeVariable!("c", PointerKind) delegate(TypeVariable!("b", PointerKind)) delegate(
        TypeVariable!("a", PointerKind)) curry(TypeVariable!("c", PointerKind) delegate(
        TypeVariable!("a", PointerKind), TypeVariable!("b", PointerKind)) f)
{
    return x => y => f(x, y);
}

/*

flip :: (a -> b -> c) -> b -> a -> c
flip f x y = f y x

*/

TypeVariable!("c", PointerKind) delegate(TypeVariable!("a", PointerKind)) delegate(
        TypeVariable!("b", PointerKind)) flip(TypeVariable!("c", PointerKind) delegate(
        TypeVariable!("b", PointerKind)) delegate(TypeVariable!("a", PointerKind)) f)
{
    return x => y => f(y)(x);
}

// levity polymorphism for free!!
TypeVariable!("a", Kind) templateIdentity(Kind)(TypeVariable!("a", Kind) x)
{
    return x;
}

void main()
{
    import std.stdio;

    auto boxed = new int(5);
    auto same = (&identity).specialize!("a", int*)()(boxed);
    writeln(*same);

    int* subtract(int* a, int* b)
    {
        return new int(*a - *b);
    }

    auto curriedSubtract = (&curry).specialize!("a", int*)
        .specialize!("b", int*)
        .specialize!("c", int*)()(&subtract);
    auto six = curriedSubtract(new int(8))(new int(2));
    writeln(*six);
}

This has been superseded: code.dlang.org/packages/erasure