Tag Archives: generic programming

Concepts: Typeclasses for C++?

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I’ve had a hypothesis for a while that C++ templates (paired at times with ADL) are an ad-hoc, unsound version of typeclasses. I’ve seen this hold for parser combinators, range base algorithms, and more. I’m also not the first to draw this comparison[1].

Concepts are supposed to bring soundness in through constrained templates. Concepts look awfully a lot like type classes; they export functions and types, and are parameterized, and act as constraints on generic functions and other concepts. I checked the draft specification [2], and it even seems to permit parameterizing concepts on type constructors, just like Haskell! (er, C++ calls them class templates, not type constructors. to-may-to, to-mah-to)

But I worried I may be mistaken about concepts, as I’ve searched through google and literature and have yet to find a single example in literature demonstrating the use of template template concepts.

In case you’re curious what this might look like, here’s an educated guess:

concept Monad<template <> class m> {
  template<typename T, typename U>
  m<U> mbind(m<T>, function<m<U>(T)>);
 
  template<class T>
  m<T> mreturn(T);
};
 
template <template <typename> class m,
          class T,
          class U,
          class Iter>
requires M<m>
requires InputIterator<Iter, U>
m<T> foldM(Iter begin, Iter end, T i, function<m<T>(T, U)> f)
{
    if(begin == end)
        return mreturn(i);
    else
        return mbind(f(i, *begin), [=](T result){
            Iter next = begin;
            ++next;
            return foldM(next, end, result, f);
        });
}

Have template template concepts been covered thoroughly somewhere, and I’ve just missed it?