type families: difference in type-checking between compile-time and run-time

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

Trac metadata

Trac field	Value
Version	6.8
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	aslatter@gmail.com
Operating system
Architecture