ghc 7.10.2 type families extension doesn't work

问题

I'm getting parse error on input ‘where’ when trying the following example in GHC 7.10.2:

{-# LANGUAGE TypeFamilies #-}

type family F a :: *
type instance where
  F (Maybe Int)  = Int
  F (Maybe Bool) = Bool
  F (Maybe a)    = String

Same question was asked two years ago about GHC 7.4.2.

I used type families fine with GHC 7.6.* (can't remember last digit) this year. Is it a problem with GHC 7.10.2?

According to the User's Guide, type families are available.

My GHC 7.10.2 and cabal 1.22.6.0 where installed from this PPA.

回答1:

You can't mix open and closed type family syntax like this, and I'm not sure where you are getting that idea from. The correct syntax is

type family F a :: * where
  F (Maybe Int)  = Int
  F (Maybe Bool) = Bool
  F (Maybe a)    = String

来源：https://stackoverflow.com/questions/32211822/ghc-7-10-2-type-families-extension-doesnt-work