Dave Bayer
test1 :: Box a -> a -> [a] test1 box x = go box x where go :: Box a -> a -> [a] go b y = [(val b), y]
The type signature "go :: Box a -> a -> [a]" is equivalent to "go :: Box b -> b -> [b]" or "go :: forall a. Box a -> a -> [a]" or "go :: forall b. Box b -> b -> [b]" because the type variable "a" in the type signature for "test1" does not scope over the type signature for "go". Fortunately, "go" here does have that type.
test2 :: Box a -> a -> [a] test2 box x = go x where -- go :: a -> [a] go y = [(val box), y]
Here "go" does not have the type "forall a. a -> [a]"; it is not polymorphic enough. To write the signature for "go" inside "test2", you need lexically scoped type variables: http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extension... -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig 2008-11-20 Universal Children's Day http://unicef.org/ 1948-12-10 Universal Declaration of Human Rights http://everyhumanhasrights.org