#7279: warning for unused type variables in instance contexts;
-fwarn-unreachable-
type-variables?
------------------------------+---------------------------------------------
Reporter: nfrisby | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.6.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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I just spend 90 minutes tracking down what ended up being a typo
introduced via find-and-replace.
I accidentally introduced a spurious constraint on an instance context:
{{{
instance (Monoid m, Context t) => Class t where …
}}}
I would like to be warned about constraints on variables that certainly
have no connection to variables in the instance head. {{{-fwarn-
unreachable-type-variables}}}?
This might also help catch those signatures where all occurrences of type
variable are as index arguments to a type family, rendering the functional
unusable because of "ambiguous type variable" errors at every call site.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7279>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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