Better error message for missing Rank2Types (or RankNTypes) flag
For a rank-2 type signature when the Rank2Types language option isn't enabled, GHC gives:
Illegal polymorphic or qualified type: (Data a) => a -> RnM a
In the type signature for `rnVarsIn':
rnVarsIn :: ((Data a) => a -> RnM a)
-> [Vdef] -> Exp -> S.Set Var -> RnM ([Vdef], Exp)It would be good to suggest using -XRank2Types or -XRankNTypes. (Sorry if this is already fixed; I don't have a working HEAD to test it with.)
Trac metadata
| Trac field | Value |
|---|---|
| Version | 6.8.2 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | Unknown |
| Architecture | Unknown |