#714: inconsistency between handling of class and signature constraints
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Reporter: [EMAIL PROTECTED] | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.5
Severity: normal | Keywords:
Os: Unknown | Difficulty: Unknown
Architecture: Unknown |
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the user's guide claims
7.4.2 3. There are no restrictions on the context in a class declaration
(which introduces superclasses), except that the class hierarchy must be
acyclic.
and we also have
7.4.3.1 1. Each universally quantified type variable tvi must be reachable
from type
suggesting that FDs are taken into account when considering reachability
but that only seems to work for signature, not for class constraints,
as the attached example shows. I wouldn't go so far as claiming this
as a bug, but it prevents an otherwise straightforward encoding of
ATS in FDs (where the superclass encodes the type function, see last
month's discussion on haskell-cafe).
[grr, there isn't a button for attachment on this form, so I'll paste the
code]
{{{
{-# OPTIONS_GHC -fglasgow-exts #-}
class C a b | a -> b
class C a b => D a
f :: C a b => a
f = undefined
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/714>
GHC <http://www.haskell.org/ghc/>
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