#5248: Infer type context in a type signature
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Reporter: gidyn | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.0.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: GHC rejects valid
program
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Comment(by gidyn):
I'm not following; perhaps I'm thinking too much like an OO programmer.
To use a simpler example,
{{{
data Pair a = Eq a => Pair {x::a, y::a}
equal :: Eq a => Pair a -> a -> Bool
equal pair z = (x pair) == (y pair) && (x pair) == z
}}}
Whenever we see Pair a, we know that Eq a. Nevertheless, the type
signature for equal must be augmented with a redundant Eq a => Pair a. If
I have two dozen functions relying on the Eq-ness of Pair a, that's two
dozen redundant type contexts.
Eq a => Pair should not need to be inferred from the definition of equal,
as it is stated explicitly in the definition of Pair.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5248#comment:6>
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