On Thu, May 6, 2010 at 9:56 AM, Eugene Dzhurinsky
On Wed, May 05, 2010 at 02:54:27PM -0700, Ryan Ingram wrote:
ErrorT is just a newtype wrapper, changing the order/application of the type variables.
newtype ErrorT e m a = ErrorT (m (Either e a)) runErrorT (ErrorT action) = action
This gives the bijection:
ErrorT :: m (Either e a) -> ErrorT e m a runErrorT :: ErrorT e m a -> m (Either e a)
That syntax is not clear for me - so ErrorT is some sort of function (calculation), which takes a monad with type (Either e a) and produces type ErrorT e m a ? Basically, i don't understand what does "ErrorT ::" means - it should name the function - but it starts with capital letter?
A constructor can be seen as a function that takes some parameters and produces a value for example with the type Maybe a, which has 2 constructors ; Just and Nothing : Prelude> :t Just Just :: a -> Maybe a the constructor Just is a function that takes a value of type a and makes a value of type Maybe a. Prelude> :t Just Just :: a -> Maybe a David.