#5248: Infer type context from type or data constructor
---------------------------------+------------------------------------------
Reporter: gidyn | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.0.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
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If I have code such as
{{{
class Foo f where
foo :: a -> f a
data Bar f a = Foo f => Bar {bar :: f a}
instance Foo (Bar f) where
foo a = Bar (foo a)
}}}
GHC will demand Foo f => on the instance declaration, even though this can
be inferred from the definition of Bar.
I understand ''why'' this is happening, but it should not be necessary to
repeat information already given. Some code violates DRY dozens of times
because of this limitation.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5248>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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