#2994: give better error messages for instance declarations with the wrong
number
of parameters
-----------------------------+----------------------------------------------
Reporter: rwbarton | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 6.11 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
-----------------------------+----------------------------------------------
Consider the following erroneous program which tries to define an instance
of `MonadReader`.
{{{
import Control.Monad.Reader
newtype Reader' r a = Reader' (r -> a)
instance MonadReader (Reader' r)
}}}
This instance declaration is wrong, because the `MonadReader` type class
has two parameters. But ghc's error message is not very helpful:
{{{
/tmp/err.hs:5:21:
`Reader' r' is not applied to enough type arguments
Expected kind `*', but `Reader' r' has kind `* -> *'
In the instance declaration for `MonadReader (Reader' r)'
}}}
While if I give too many arguments, e.g., `instance MonadReader r r
(Reader' r)`, I always get a good error message, even if the kinds of some
of the leading types are wrong:
{{{
Kind error: `MonadReader' is applied to too many type arguments
In the instance declaration for `MonadReader r r (Reader' r)'
}}}
So it'd be great if the error message for giving too few type arguments
could be more like that one.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2994>
GHC <http://www.haskell.org/ghc/>
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