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0 pointsmoomin8mo ago0 comments

Your basic problem is that your programming language can’t express the concept cleanly. You need what’s called “Higher-Kinded Types”.

To give you a concrete example, in C#

Func<A, B>, List<A> -> List

Func<A, B>, Task<A> -> Task

Func<A, B>, Func<C, A> -> Func<C, B>

Can’t be expressed using a generalisation. But in Haskell, you can write

(Functor F) => Func<A,B>, F<A> -> F

One of the biggest things that makes monads hard to understand is that the type systems of most languages can’t represent them. Annoying, that includes the “typeless” ones.

0 comments

WorldMaker8mo ago

C# is a fun example because there is ongoing work to support Higher-Kinded Types in it: https://paullouth.com/higher-kinds-in-c-with-language-ext/

jameshart8mo ago

I'm sorry, I'm not sure I understand entirely what you are trying to express by

  Func<A, B>, List<A> -> List<B>

That said, in C#, you can write:

  List<A> listA;
  Task<A> taskA;
  Func<A, B> func;
  
  List<B> listB = from i in listA select func(i);
  Task<B> taskB = from t in taskA select func(t);

And if it can resolve a method on List<T> called 'Select' that takes a Func<T, S> that returns a List<S>, and a method on Task<T> called 'Select' that takes a Func<T, S> that returns a Task<S> this will compile.

In other words, I kind of think that Select methods (which can be Select extension methods, of course) amount to functors in C#?

instig0078mo ago

now write a single function that performs

  from x in xs select fn(x)

and define a signature for it where `xs` and `fn` are the only input arguments, so that it accepts both `listB` and `taskB` without a compilation error.

gpderetta8mo ago

In C++:

    template<template<class> class Container, class T, invocable<T> Fn>
    Container<invoke_result_t<Fn, T> > map(Container<T> container, Fn fn) {
      return to<Container>(transform(container,  fn));
    }

(Although idiomatically you wouldn't write the code this way).

Whether that counts as HK types, I'll leave it to others to discuss.

jameshart8mo ago

Right, ‘some type that has a Select(Func<T, S>) method available’ is not a thing you can express in C# generic constraints.

But I don’t need a function that does that, the language has syntax for it - I can just do that wherever I need it.

2 more replies

hollerith8mo ago

>One of the biggest things that makes monads hard to understand is that the type systems of most languages can’t represent them. Annoying, that includes the “typeless” ones.

Well, yeah, since a monad is a type, then a "typeless" PL will not be able to represent it.

j / k navigate · click thread line to collapse

0 pointsmoomin8mo ago0 comments

Your basic problem is that your programming language can’t express the concept cleanly. You need what’s called “Higher-Kinded Types”.

To give you a concrete example, in C#

Func<A, B>, List<A> -> List

Func<A, B>, Task<A> -> Task

Func<A, B>, Func<C, A> -> Func<C, B>

Can’t be expressed using a generalisation. But in Haskell, you can write

(Functor F) => Func<A,B>, F<A> -> F

One of the biggest things that makes monads hard to understand is that the type systems of most languages can’t represent them. Annoying, that includes the “typeless” ones.

0 comments

WorldMaker8mo ago

C# is a fun example because there is ongoing work to support Higher-Kinded Types in it: https://paullouth.com/higher-kinds-in-c-with-language-ext/

jameshart8mo ago

I'm sorry, I'm not sure I understand entirely what you are trying to express by

  Func<A, B>, List<A> -> List<B>

That said, in C#, you can write:

  List<A> listA;
  Task<A> taskA;
  Func<A, B> func;
  
  List<B> listB = from i in listA select func(i);
  Task<B> taskB = from t in taskA select func(t);

In other words, I kind of think that Select methods (which can be Select extension methods, of course) amount to functors in C#?

instig0078mo ago

now write a single function that performs

  from x in xs select fn(x)

and define a signature for it where `xs` and `fn` are the only input arguments, so that it accepts both `listB` and `taskB` without a compilation error.

gpderetta8mo ago

In C++:

    template<template<class> class Container, class T, invocable<T> Fn>
    Container<invoke_result_t<Fn, T> > map(Container<T> container, Fn fn) {
      return to<Container>(transform(container,  fn));
    }

(Although idiomatically you wouldn't write the code this way).

Whether that counts as HK types, I'll leave it to others to discuss.

jameshart8mo ago

Right, ‘some type that has a Select(Func<T, S>) method available’ is not a thing you can express in C# generic constraints.

But I don’t need a function that does that, the language has syntax for it - I can just do that wherever I need it.

2 more replies

hollerith8mo ago

>One of the biggest things that makes monads hard to understand is that the type systems of most languages can’t represent them. Annoying, that includes the “typeless” ones.

Well, yeah, since a monad is a type, then a "typeless" PL will not be able to represent it.

j / k navigate · click thread line to collapse