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let myMap = fix (\map' f list -> case list of [] -> []; (x:xs) -> f x : map' f xs) in myMap (+1) [1..]

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dropAt :: Int -> [a] -> [a]
dropAt = go id
  where
    go :: ([b] -> [b]) -> Int -> [b] -> [b]
    go f n [] = f []
    go f 0 (x:xs) = f xs
    go f n (x:xs) = go (f . (x:)) (n-1) xs

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splitInHalf :: [a] -> ([a], [a])
splitInHalf [] = ([], [])
splitInHalf xs = go id xs xs
  where
    go :: ([b] -> [b]) -> [b] -> [b] -> ([b], [b])
    go f xs [] = (f [], xs)
    go f xs [_] = (f [], xs)
    go f (x:xs) (_:_:ys) = go (f . (x:)) xs ys

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{-# LANGUAGE OverloadedStrings #-}

import Control.Concurrent.Async
import Control.Concurrent.QSem
import Control.Monad
import Control.Monad.Catch
import Control.Monad.Trans
import Control.Monad.Reader

import Data.ByteString.Lazy (ByteString)

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Authors

Merijn Verstraaten

Date

2014/11/22

So You Want to Be a Super Cool GHC Hacker?

So you have a pet peeve/bug/feature request that you'd like to see added to GHC. You made sure there was a Trac ticket for it, but despite your patient waiting no one is solving your problem. Those selfish GHC hackers!

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{-# LANGUAGE ForeignFunctionInterface #-}
module Foo where

import Foreign.C.String
import Foreign.C.Types

foreign export ccall start_server :: CInt -> CString -> IO ()
foreign export ccall start_client :: CString -> IO ()

start_server :: CInt -> CString -> IO ()

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module RankN where

data Foo = Foo Int | Bar Double

mangle :: Num a => (a -> a) -> Foo -> Foo
mangle f (Foo i) = Foo (f i)
mangle f (Bar d) = Bar (f d)

{-
NoRankN.hs:6:25:

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

import Prelude hiding (head, tail, zip)

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

import GHC.Prim (Constraint)

data Nat = Succ Nat | Zero

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{-# 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