Wrong diagnostics for instance signature mismatch
Consider:
# LANGUAGE InstanceSigs, DataKinds, KindSignatures, GADTs #
data DayKind = Work | Rest
data Day :: DayKind -> * where
Mon :: Day Work Sat :: Day Rest
instance Num (Day c) where
fromInteger :: Int -> Day p fromInteger 1 = Mon fromInteger 6 = Sat
I get
$ ~/bin/ghci NumGADT.hs GHCi, version 7.5.20120410: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( NumGADT.hs, interpreted )
NumGADT.hs:10:18:
Method signature does not match class; it should be
| fromInteger | forall (p :: DayKind). Int -\> Day p |
|---|---|
| In the instance declaration for \`Num (Day c)' |
Failed, modules loaded: none. Prelude>
It appears to me that instead of telling me the expected signature GHCi simply echoes what I have provided. This is not really useful ;-)
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.5 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |