GHC does not accept GADT type signature

Question

This declaration doesn't work:

data Identity a where {Identity :: (->) a (Identity a)}

How to fix this?

Answer 1

At least as of GHC 7.8, if you turn on GADT support, then your code typechecks and does what you'd expect :

{-# LANGUAGE GADTs #-}
data Identity a where {Identity :: (->) a (Identity a)}

resulting in:

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             ( foo.hs, interpreted )
Ok, modules loaded: Main.
λ» :i Identity
data Identity a = Identity a    -- Defined at foo.hs:2:1