instance (EmbedAsChild m c, m1 ~ m) => EmbedAsChild m (XMLGenT m1 c)
That looked to me like a long-winded way of saying:
instance (EmbedAsChild m c) => EmbedAsChild m (XMLGenT m c)
Unless I'm missing something?
These two instances are not equivalent: - the first matches even if m and m1 differ, causing a type-error. - the second matches only if m~m1 Claus {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE MultiParamTypeClasses #-} class C a b where c :: a -> b -> Bool instance C a a where c _ _ = True instance C a b where c _ _ = False class D a b where d :: a -> b -> Bool instance a~b=>D a b where d _ _ = True -- instance D a b where d _ _ = False -- would be a duplicate instance {- *Main> c () () True *Main> c () True False *Main> d () () True *Main> d () True <interactive>:1:0: Couldn't match expected type `Bool' against inferred type `()' When generalising the type(s) for `it' -}