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The following code:

{-# LANGUAGE DataKinds      #-}
{-# LANGUAGE GADTs          #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeOperators  #-}

import GHC.TypeLits

data Vec :: Nat -> * -> * where
  Nil  :: Vec 0 a
  (:>) :: a -> Vec n a -> Vec (n + 1) a

infixr 5 :>

when loaded in GHCi 7.8.3, and asking for the type of (1 :> 2 :> 3 :> Nil), gives:

$ ghci example/vec.hs 
GHCi, version 7.8.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( example/vec.hs, interpreted )
Ok, modules loaded: Main.
*Main> :t (3:>4:>5:>Nil)
(3:>4:>5:>Nil) :: Num a => Vec 3 a

while in GHCi 7.10.1 it gives:

$ ghci example/vec.hs 
GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( example/vec.hs, interpreted )
Ok, modules loaded: Main.
*Main> :t (3:>4:>5:>Nil)
(3:>4:>5:>Nil) :: Num a => Vec (2 + 1) a

That is, the type-level computation, ((0 + 1) + 1) + 1 is only simplified to 2 + 1 in GHC 7.10.1, whereas in 7.8.3 2+1 was further simplified to 3.

The same still happens in HEAD (20150417)

$ ghci example/vec.hs 
GHCi, version 7.11.20150417: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( example/vec.hs, interpreted )
Ok, modules loaded: Main.
*Main> :t (3:>4:>5:>Nil)
(3:>4:>5:>Nil) :: Num a => Vec (2 + 1) a

I strongly feel that the behaviour in ghc 7.8.3 is the desired behaviour.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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