Confusing type error from constrained class method
import GHC.Stack
class Foo a where
foo :: HasCallStack => a -> String
instance Foo () where
foo () = prettyCallStack callStack
main = putStrLn (foo ())
~/S/ghc> ./inplace/bin/ghc-stage2 --make Foo.hs
[1 of 1] Compiling Main ( Foo.hs, Foo.o )
Foo.hs:4:3: error:
• Constraint ‘HasCallStack’ in the type of ‘foo’
constrains only the class type variables
Use ConstrainedClassMethods to allow it
• When checking the class method:
foo :: forall a. (Foo a, HasCallStack) => a -> String
In the class declaration for ‘Foo’
The error says that HasCallStack
constrains the class type variable a
, which is incorrect. Is the error message supposed to be that HasCallStack
should constrain the class type variable and nothing else?
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1-rc2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |