#3169: Type families occurs check
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Reporter: simonpj | Owner: chak
Type: bug | Status: new
Priority: normal | Milestone: 6.12.1
Component: Compiler | Version: 6.10.2
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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Consider this:
{{{
{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
module Map where
import Prelude hiding ( lookup )
class Key k where
type Map k :: * -> *
lookup :: k -> Map k elt -> Maybe elt
instance (Key a, Key b) => Key (a,b) where
type Map (a,b) = MP a b
lookup (a,b) (m :: Map (a,b) elt)
= case lookup a m :: Maybe (Map b elt) of
Just (m2 :: Map b elt) -> lookup b m2 :: Maybe elt
data MP a b elt = MP (Map a (Map b elt))
}}}
This ought to typecheck, even in the absence of all those type signatures.
But alas:
{{{
Map.hs:13:12:
Occurs check: cannot construct the infinite type: elt = t elt
In the expression: lookup a m :: Maybe (Map b elt)
In the expression:
case lookup a m :: Maybe (Map b elt) of {
Just (m2 :: Map b elt) -> lookup b m2 :: Maybe elt }
}}}
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3169>
GHC <http://www.haskell.org/ghc/>
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