[GHC] #1809: type families: difference in type-checking between compile-time and run-time

[GHC]

#1809: type families: difference in type-checking between compile-time and run-
time
-----------------------+----------------------------------------------------
  Reporter:  guest     |          Owner:       
      Type:  bug       |         Status:  new  
  Priority:  normal    |      Milestone:       
 Component:  Compiler  |        Version:  6.8  
  Severity:  normal    |       Keywords:       
Difficulty:  Unknown   |             Os:  Linux
  Testcase:            |   Architecture:  x86  
-----------------------+----------------------------------------------------
 in the module
 {{{
 {-# LANGUAGE TypeFamilies #-}

 module Main where

 import Prelude hiding (head)
 import qualified Prelude

 type family Elem a :: *

 class ListLike t where
         head :: t -> Elem t

 -- Instances

 type instance Elem [a] = a

 instance ListLike [a] where
         head = Prelude.head

 x = head [1 :: Int,2,3]

 main = putStrLn . show $ x
 }}}

 Here is the resulting interactive session, which I think is weird:

 {{{
 $ ghci Main
 GHCi, version 6.9.20071027: http://www.haskell.org/ghc/  :? for help
 Loading package base ... linking ... done.
 [1 of 1] Compiling Main             ( Main.hs, interpreted )
 Ok, modules loaded: Main.
 *Main> :t x
 x :: Elem [Int]
 *Main> putStrLn . show $ x

 <interactive>:1:11:
     No instance for (Show (Elem [Int]))
       arising from a use of `show' at <interactive>:1:11-14
     Possible fix: add an instance declaration for (Show (Elem [Int]))
     In the second argument of `(.)', namely `show'
     In the first argument of `($)', namely `putStrLn . show'
     In the expression: putStrLn . show $ x
 *Main> main
 1
 }}}

 Behavior is similar in GHC version 6.8.0.20071028

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1809>
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