#1432: Poor type error messages (refer to generated code rather than the higher
level problem)
--------------------------------------------+-------------------------------
Reporter: igloo | Owner:
Type: bug | Status: new
Priority: high | Milestone: 6.8
Component: Compiler (Type checker) | Version: 6.6.1
Severity: normal | Keywords:
Difficulty: Unknown | Os: Unknown
Testcase: tcfail046,tcfail169,tcfail118 | Architecture: Unknown
--------------------------------------------+-------------------------------
With this module:
{{{
module Foo where
data Foo a = Foo
data Bar a = Bar (Foo a)
deriving Eq
}}}
the HEAD says:
{{{
q.hs:5:0:
Non type-variable argument in the constraint: Eq (Foo a)
(Use -fglasgow-exts to permit this)
In the context: (Eq (Foo a))
While checking the context of an instance declaration
In the derived instance: (Eq (Foo a)) => Eq (Bar a)
}}}
which isn't really the error message that we want, as it's refering to the
generated instance.
6.6 gives the much better:
{{{
q.hs:5:5:
No instance for (Eq (Foo a))
arising from the 'deriving' clause of a data type declaration
at q.hs:5:5
Possible fix: add an instance declaration for (Eq (Foo a))
When deriving the instance for `Eq (Bar a)'
}}}
This example is from tcfail046; tcfail169 is similar and tcfail118 is also
a related issue.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1432>
GHC <http://www.haskell.org/ghc/>
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