GHC不接受GADT类型签名

Question

该声明不起作用:

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

如何解决这个问题？

Answer 1

至少从 GHC 7.8 开始，如果您打开 GADT 支持，那么您的代码将进行类型检查并执行您期望的操作：

\n\n

{-# LANGUAGE GADTs #-}\ndata Identity a where {Identity :: (->) a (Identity a)}\n

\n\n

导致：

\n\n

GHCi, version 7.8.3: http://www.haskell.org/ghc/  :? for help\nLoading package ghc-prim ... linking ... done.\nLoading package integer-gmp ... linking ... done.\nLoading package base ... linking ... done.\n[1 of 1] Compiling Main             ( foo.hs, interpreted )\nOk, modules loaded: Main.\n\xce\xbb\xc2\xbb :i Identity\ndata Identity a = Identity a    -- Defined at foo.hs:2:1\n

\n