On 13 January 2011 13:03, z_axis <z_a...@163.com> wrote: > > data Layout a = forall l. (LayoutClass l a, Read (l a)) => Layout (l a) > > readsLayout :: Layout a -> String -> [(Layout a, String)] > readsLayout (Layout l) s = [(Layout (asTypeOf x l), rs) | (x, rs) <- reads > s] > > Why is the first parameter of readsLayout "(Layout l)" not "Layout (l a)" ?
Look at the data definition: if we replace the constraint with some "L" type of kind * -> *, we have: data Layout a = Layout (L a) Note that the type of the value contained within the actual constructor is the application of L (originally l) to a. For example, if L == Maybe, then this could be a value of type "Layout Int" with the value "Layout (Just 3)". As such, there is only one actual internal value to pattern match against. -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
