#1928: Confusing type error message
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Reporter: josef | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.8.1
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Architecture: Unknown | Os: Unknown
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The following code (which is part of a bigger module) needs scoped type
variables to compile.
{{{
run_state :: forall a s. State s a -> s -> (a,s)
run_state m s = observe_monad unit_op bind_op m where
unit_op v = (v,s)
bind_op :: BindOp (StateE s) a (a,s)
bind_op Get k = run_state (k s) s
bind_op (Put s1) k = run_state (k ()) s1
}}}
However, forgetting to turn on scoped type variables will give a very
confusing error message:
{{{
Unimo.hs:56:36:
Couldn't match expected type `s1' against inferred type `s'
`s1' is a rigid type variable bound by
the type signature for `bind_op' at Unimo.hs:55:28
`s' is a rigid type variable bound by
the type signature for `run_state' at Unimo.hs:52:22
In the first argument of `k', namely `s'
In the first argument of `run_state', namely `(k s)'
In the expression: run_state (k s) s
}}}
Line 52 is the type signature of run_state and line 55 is the type
signature of bind_op. The error message talks about a type variable `s1'
which isn't mentioned anywhere. I guess the reason for this is that we
have name collision and this is ghc's way of trying to tell the two
variables apart. I don't think it works that well though. But I'm afraid I
don't have any suggestion on how to make it better.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1928>
GHC <http://www.haskell.org/ghc/>
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