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  • #10321

Closed
Open
Opened Apr 18, 2015 by darchon@trac-darchon

GHC.TypeLits.Nat types no longer fully simplified.

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
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture
Assignee
Assign to
7.10.2
Milestone
7.10.2 (Past due)
Assign milestone
Time tracking
None
Due date
None
Reference: ghc/ghc#10321