#6123: occurs check should not apply to type families
------------------------------+---------------------------------------------
Reporter: dmwit | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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This code:
{{{
{-# LANGUAGE TypeFamilies #-}
type family Id a
cid :: a ~ Id a => a -> a
cid x = x
cundefined = cid undefined
}}}
gives this error:
{{{
test.hs:5:14:
Occurs check: cannot construct the infinite type: a0 = Id a0
In the expression: cid undefined
In an equation for `cundefined': cundefined = cid undefined
}}}
whereas I think it should be possible to infer the type cundefined :: a ~
Id a => a. Equations with a variable on one side and a type family at the
head of the other side should become constraints rather than being subject
to the occurs check.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6123>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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