#5296: Compile succeeds without signature, but fails with the signature
suggested
by GHC
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Reporter: dsf | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.4.1
Component: Compiler (Type checker) | Version: 7.0.3
Keywords: | Testcase:
Blockedby: 1897 | Difficulty:
Os: Linux | Blocking:
Architecture: x86_64 (amd64) | Failure: GHC rejects valid
program
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Comment(by simonpj):
The problem here is related to #1897, as you point out, but is even
clearer because it doesn't even involve type families. The trouble is
this. `method` has type
{{{
method :: C t1 t2 m => Int -> m t2
}}}
Notice that `t1` does not appear in `method`'s type. Now GHC is faced
with
{{{
From given (C t1 t2 m)
deduce wanted (C t3 t2 m)
}}}
Notice the t3. The call of `method` means that the second and third args
of C must be t2, m; but the first can be anything. So type inference is
supposed to guess what type it should use for t3. Here there is a unique
choice, but in general it is hard to solve problems where there is are
''positive'' clues, only that there is just one magic solution.
If we could supply the type arguments to the call to `method`, we could
say this:
{{{
f :: forall t1 t2 (m :: * -> *). C t1 t2 m => Int -> m t2
f x = method @t1 @t2 @m x
}}}
Here I put the type args with a leading "@" (the notation I'm currently
considering for type args). Now we'd be fine.
In short, the only Decent Solution here seems to me to be explicit type
arguments. Unless anyone else has better ideas.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5296#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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