#1549: ghc derives different type signature for equal functions with overlapping
instances
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Reporter: int-e | Owner:
Type: proposal | Status: reopened
Priority: low | Milestone:
Component: Documentation | Version: 6.7
Severity: trivial | Resolution:
Keywords: | Difficulty: Unknown
Os: Linux | Testcase:
Architecture: x86 |
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Comment (by simonpj):
Looking at it again, I this would be better to add this after "without
complaining about
the problem of subsequent instantiations":
Notice that we gave a type signature to `f` so GHC had to
''check'' that `f` has the specified type.
Suppose instead we do not give a type signature, asking GHC to ''infer''
it instead. In this case, GHC will refrain from
simplifying the constraint `C Int [Int]` (for the same reason
as before) but, rather than rejecting the program, it will infer the type
{{{
f :: C Int b => [b] -> [b]
}}}
That postpones the question of which instance to pick to the
call site for `f`
by which time more is known about the type `b`
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1549>
GHC <http://www.haskell.org/ghc/>
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