#2900: Confusing error message for monadic function with wrong number of
arguments
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Reporter: tim | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.10.1
Severity: minor | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Changes (by simonpj):
* cc: chak (added)
* difficulty: => Unknown
Comment:
Good example! Sadly, it's quite awkward to fix. Here's what is
happening.
* GHC tries to make the type `Int -> m ()` look like `ty1 -> ty2 -> ty3`
(to match the syntactic form of the LHS.
* It's ''possible'' that the context `(MonadIO m)` might have, as a
"superclass" the equality `m ~ (->) Int`. In that case we'd get `ty2 =
Int`, `ty3 = ()`.
* Because `m` might be elucidated by the context, GHC can't reject the
result type `(m ())` right away. Instead it invents a fresh type variable
alpha, and generates the contraint that `(m ~ (->) alpha)`, leaving the
constraint to be solved later.
* Now we go on to the RHS, and that gives a type error.
What we really want here is to solve the first constraint "eagerly", and
proceed only if it succeeds. The type checker isn't set up to do that at
the moment, but this is an interesting example to bear in mind, thank you.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2900#comment:1>
GHC <http://www.haskell.org/ghc/>
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