24 Jul
2009
24 Jul
'09
12:44 p.m.
On Fri, Jul 24, 2009 at 09:41:34AM -0300, Rafael Gustavo da Cunha Pereira Pinto wrote:
Hi folks,
Once in a while I stumble upon declarations like this:
class EditAction e a | e -> a where apply :: a -> e -> a
Why is the vertical bar there? I understand that => defines a context, but can't find what is the "|" for.
It's a functional dependency: the e -> a means that the type e determines the type a, that is, there can only be one instance of EditAction for any particular type e. -Brent