Compiler could warn about type variable shadowing, and hint about ScopedTypeVariables
GHC already warns about variable shadowing:
$ cat test.hs
module Test where
timesTwoPlusOne x
= timesTwo x + 1
where timesTwo x = x * 2
$ ghc -fwarn-name-shadowing test.hs
...
Warning:
This binding for `x' shadows the existing binding
bound at <location>
However the similar warning doesn't happen for type variables.
$ cat T9244.hs
module T9244 where
import Control.Exception
tryMaybe :: IO a -> IO (Maybe a)
tryMaybe action = do
result <- (try action) :: IO (Either SomeException a)
return $ case result of
Left _ -> Nothing
Right v -> Just v
$ ghc -fwarn-name-shadowing T9244.hs
...
Couldn't match type `a' with `a1'
`a' is a rigid type variable bound by
the type signature for tryMaybe :: IO a -> IO (Maybe a)
at types.hs:<line>:13
`a1' is a rigid type variable bound by
an expression type signature: IO (Either SomeException a1)
at types.hs:<line>:15
Expected type: IO a1
Actual type: IO a
...
Here, I thought that the 'a' in the function's type declaration was the same 'a' in the expression type declaration. However in Haskell 98, they are completely different variables.
Suggestion: if a type variable is renamed by the compiler due to a clash with another type variable, issue a warning that the second shadows the first, and give a hint about using -XScopedTypeVariables and forall.
Alternative suggestion: if an error is displayed, where the error contains a renamed type variable, issue a hint that the second shadows the first, and give a hint about using -XScopedTypeVariables and forall.