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{-# LANGUAGE ExistentialQuantification, DeriveDataTypeable #-}
-- This module requires quite trick desugaring,
-- because of the context in the existentials
-- It broke a pre 6.4 GHC
module Foo where
import Data.Data
import Data.HashTable
data Item = forall a. (Data a) => Leaf Bool a
| forall a. (Data a) => Branch Bool a Int Int
deriving (Typeable)
instance Data Item where
gfoldl k z (Leaf b v) = z (Leaf b) `k` v
gfoldl k z (Branch b v a1 a2) = z (\x -> Branch b x a1 a2) `k` v
gunfold _ _ _ = error "urk"
toConstr (Leaf _ _) = leafConstr
toConstr (Branch _ _ _ _) = branchConstr
dataTypeOf _ = itemDataType
itemDataType = mkDataType "Subliminal.Item" [leafConstr, branchConstr]
leafConstr = mkConstr itemDataType "Leaf" [] Prefix
branchConstr = mkConstr itemDataType "Branch" [] Prefix
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