#7288: type inference fails with where clause (RankNTypes, TypeFamilies)
---------------------------------------+------------------------------------
Reporter: agrafix | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type
checker)
Version: 7.6.1 | Keywords: type inference,
rankntypes, typefamilies
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
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when using RankNTypes and TypeFamilies with polymorphic functions, type
inference works in let but not in where clauses.
{{{
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}
import Control.Monad
class Monad m => GenM m where
type GenReq m :: *
type GenFun a b = forall m. GenM m => a -> m b
data GenDef a b = GenDef { name :: String, fun :: GenFun a b }
data Gen a b = Gen
data Struct = Struct
mkGen :: forall a b. String -> GenFun a b -> GenDef a b
mkGen = GenDef
runGen :: GenM m => Gen a b -> a -> m (Maybe b)
runGen = undefined
getPatients :: GenM m => m [Int]
getPatients = undefined
-- BROKEN
myGen :: Gen String Struct -> GenDef Int [Struct]
myGen structGen =
mkGen "myGen" $ \pid ->
do pats <- getPatients
structs <- mapM patToStruct pats
return structs
where
patToStruct pid =
do Just struct <- runGen structGen (show pid)
return struct
-- WORKS
myGen' :: Gen String Struct -> GenDef Int [Struct]
myGen' structGen =
mkGen "myGen" $ \pid ->
do pats <- getPatients
let patToStruct pid =
do Just struct <- runGen structGen (show pid)
return struct
structs <- mapM patToStruct pats
return structs
-- WORKS
myGen'' :: Gen String Struct -> GenDef Int [Struct]
myGen'' structGen =
mkGen "myGen" $ \pid ->
do pats <- getPatients
structs <- mapM patToStruct pats
return structs
where
patToStruct :: GenM m => Int -> m Struct
patToStruct pid =
do Just struct <- runGen structGen (show pid)
return struct
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7288>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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