Type checker does not use functional dependency to avoid ambiguity
Compiling the following module gives an error
module X where
class C a | -> a
instance C Int
unC :: (C a) => a -> Int
unC i = undefined
test :: Int
test = unC undefinedError message:
X.hs:13:7:
Ambiguous type variable `a' in the constraint:
`C a' arising from a use of `unC' at X.hs:13:7-19
Probable fix: add a type signature that fixes these type variable(s)But that is just plain wrong. The functional dependency in the definition of C forces a to be Int. No other type is possible. So what's ambiguous about that?
Trac metadata
| Trac field | Value |
|---|---|
| Version | 6.7 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | lennart@augustsson.net |
| Operating system | MacOS X |
| Architecture | x86 |