Custom Query (7491 matches)
Results (1  3 of 7491)
Ticket  Resolution  Summary  Owner  Reporter 

#10128  fixed  Data.List.transpose needs more docs  ekmett  nomeata 
Description 
as mentioned by Doug McIlroy on haskellprime. My preference in the interest of brevity would be to not include the equations that Doug mentioned and simply add his nicely constructed educating example to the docs:


#10125  invalid  Improper evaluation of type families  danilo2  
Description 
Hello! I've got here a small piece of code: {# LANGUAGE DeriveDataTypeable #} {# LANGUAGE EmptyDataDecls #} {# LANGUAGE DataKinds #} {# LANGUAGE PolyKinds #} {# LANGUAGE TypeFamilies #} {# LANGUAGE TypeOperators #} {# LANGUAGE UndecidableInstances #} module Data.TypeLevel.Bool where import Prelude hiding (Eq) import qualified Prelude as P import Data.Typeable import GHC.TypeLits type family Eq (a :: k) (b :: k) where Eq a a = True Eq a b = False type family If (cond :: Bool) (a :: k) (b :: k) where If True a b = a If False a b = b type family CmpBy (f :: k > k > Ordering) (a :: [k]) (b :: [k]) :: Ordering where CmpBy f '[] '[] = EQ CmpBy f (a ': as) (b ': bs) = If (Eq (f a b) EQ) (CmpBy f as bs) (f a b) type family TCompare (a :: [Nat]) (b :: [Nat]) :: Ordering where TCompare '[] '[] = EQ TCompare (a ': as) (b ': bs) = If (Eq a b) (TCompare as bs) (CmpNat a b) type N1 = '[1,2,3,5] type N2 = '[1,2,3,4] main = do print $ (Proxy :: Proxy GT) == (Proxy :: Proxy (TCompare N1 N2)) print $ (Proxy :: Proxy GT) == (Proxy :: Proxy (CmpBy CmpNat N1 N2)) It does NOT compile, while it should. The typelevel functions TCompare and CmpBy CmpNat should work exactly the same way. Right now the former one always returns EQ, so the program does not compile with an error: Bool.hs:49:41: Couldn't match type ‘'EQ’ with ‘'GT’ Expected type: Proxy (CmpBy CmpNat N1 N2) Actual type: Proxy 'GT In the second argument of ‘(==)’, namely ‘(Proxy :: Proxy (CmpBy CmpNat N1 N2))’ In the second argument of ‘($)’, namely ‘(Proxy :: Proxy GT) == (Proxy :: Proxy (CmpBy CmpNat N1 N2))’ 

#10123  invalid  GHCi won't start in Powershell ISE  qwfy  
Description 
To reproduce,
You will see the output: GHCi, version 7.8.3: http://www.haskell.org/ghc/ :? for help Loading package ghcprim ... linking ... done. Loading package integergmp ... linking ... done. Loading package base ... linking ... done. After Loading package base ... linking ... done., GHCi just hangs, won't continue, the prompt is never displayed. 