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 |