scope - Haskell Referencing a Type Variable -

i run problem , wanted ask if there's common solution or pattern. is possible make type variable in nested context reference type outer context? example,

foo :: -> ... -> .. foo = ...    bar :: -> ... 

now bar's a different foo's a. typically want, makes life difficult, , need make them same. i've used dirty tricks force type checker unify 2 in past, thwarted. here's latest example (a parsec function) spurred me asking question.

data project = ... deriving enum data stuff = ...  pproject :: monad m => p m stuff pproject =   stuff <- pstuff   ...   convert stuff <$> penum :: p m project  penum :: (monad m, enum a) => string -> p m penum = ... 

the convert function needed type, hence had specify annotation :: p m project. however, means have introduce m, unfortunately not same m in function signature. type checker reports with:

could not deduce monad m1 arising use of penum context monad m

is there way reference function signature's m without ugly hack? (an ugly hack inserting dummy code doesn't executed, exists unify 2 types.)

you're looking scopedtypevariables extension, lets reference type variables containing scopes.

{-# language scopedtypevariables #-} 

for backwards compatibility, applies type signatures have explicit forall. have write:

pproject :: forall m. monad m => p m stuff 

after that, you'd able refer correct type variable m inside scope of pproject.