merijn

The Cabal/Stack Disambiguation Guide

One of the most frequently asked Haskell beginner questions in recent years is:

"Stack or cabal?"

I will helpfully not answer this question. Instead I will hope to eliminate the confusion that many of the askers seem to have about the various different

merijn

Why You Should Stop Saying "Concrete Type"!

TL;DR: There is no concrete (hah!) definition of "concrete type", different people are meaning different things by those words and assuming everyone else means the same thing.

A Quick Type Primer

merijn

import Foreign

main :: IO Int
main = peek nullPtr

merijn

#include <stdio.h>

char X = 'a';

int main(int argc, char* argv[])
{
    (void) argc;
    (void) argv;

    struct X {

merijn

data MyState s a = MyState (s -> (a, s))

get :: MyState s s
get = undefined

put :: s -> MyState s ()
put = undefined

modify :: (s -> s) -> MyState s ()
modify = undefined

merijn

#include <stdio.h>

int main(int argc, char **argv)
{
    int d = sizeof(0)["ABCDEFGHIJKLMNOPQRSTUWVXYZ"];
    printf("%d\n", d);
    return 0;
}

merijn

{-# LANGUAGE ScopedTypeVariables #-}
module Main where

import Control.Monad (forM_)
import Data.Maybe (fromMaybe)

generalisedFizzBuzz
    :: forall a f m
     . (Foldable f, Semigroup m)
    => (a -> m) -> (a -> a -> Bool) -> f (a, m) -> a -> m

merijn

{-# LANGUAGE ConstraintKinds, DataKinds, PolyKinds, TypeFamilies, TypeOperators #-}

import GHC.Exts (Constraint)

type family Restrict (a :: k) (as :: [k]) :: Constraint where
    Restrict a (a ': as) = ("Error!" ~ "Tried to apply a restricted type!")
    Restrict x (a ': as) = Restrict x as
    Restrict x '[] = ()

foo :: Restrict a [(), Int] => a -> a

merijn

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}

import GHC.Prim (Constraint)

data Nat = Succ Nat | Zero

merijn

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
module HList where

import Prelude hiding (head, tail, zip)