haskell-jp / questions #23

class Comonoid a where
  coidentity :: a -> ()
  comultiple :: a -> (a, a)

class Comonoid a where
    v :: a -> ()
    p :: a -> a
    q :: a -> a

forall x.

((p.p) x, (q.p) x , q x) = (p x, (p.q) x, (q.q) x)

p x = x
q x = x

instance Comonoid a where
  coidentity _ = ()
  comultiple x = (x, x)

import Data.Type.Equality

unProd :: a1 :~: a2 -> b1 :~: b2 -> '(a1, b1) :~: '(a2, b2)
unProd r1 r2 = undefined -- Refl

unProd :: a1 :~: a2 -> b1 :~: b2 -> '(a1, b1) :~: '(a2, b2)
unProd Refl Refl = Refl

extractedTuple' :: forall a b t. (t ~ '(a, b)) => '(Fst t, Snd t) :~: t
extractedTuple' = apply (apply Refl (Refl :: Fst t :~: a)) (Refl :: Snd t :~: b)

{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeOperators #-}

module Foo where

import Data.Singletons.Prelude (Fst, Snd)
import Data.Type.Equality ((:~:)(Refl))

extractedTuple :: forall a fst snd. (fst ~ Fst a, snd ~ Snd a, a ~ '(fst, snd)) => '(Fst a, Snd a) :~: a
extractedTuple = Refl

useExtractedTuple :: ()
useExtractedTuple =
  case extractedTuple @'(Int, Bool) of
    Refl -> ()

hoge :: forall a. a -> [a] -> [a]
hoge x = go
  where
    go :: [a] -> [a]
    go xs = x : xs

h <- getHomeDir
readFile $ toFilePath $ h </> $(mkRelFile ".bashrc")

Modifying data through the ForeignPtr afterwards is unsafe if the vector could have been frozen before the modification.

-- evilFunを使用する人には"hiroto"がエラーを吐くことがわからない。
evilFun :: Text -> Text
evilFun "hiroto" = error "This is evil function"
evilFun _ = "Everything is alright!"

evaluate :: a -> IO a

import Data.Vector.Storable.Mutable
import Data.Vector.Storable (freeze)
import 
import Foreign.ForeignPtr
import Foreign.Storable
import Data.Foldable

main :: IO ()
main = do
  fp <- mallocForeignPtrArray 10
  withForeignPtr fp $ \p -> flip traverse_ [0..10] $ \i -> do
    pokeElemOff p i i
  let nv = runST $ freeze (unsafeFromForeignPtr0 fp 10)
  withForeignPtr fp $ \p -> do
    poke p 42
    pure ()
  print nv
  pure ()