On 24 May 2009, at 01:19, Max Cantor wrote:

Going through the instances for HTypeable (http://www.haskell.org/HaXml/HaXml/src/Text/XML/HaXml/TypeMapping.html#toHTy... ) I saw the following instance for Either a b.

My question is, why doesn't the pattern match in the where clause always fail? If (Left x) = m does not fail, doesn't that imply that m is a Left x and therefore the (Right y) = m should fail?

It does; at least one of x and y is (_|_). However, toHType function doesn't really need it's argument; all that is needed is the type of an argument. Therefore, toHType is quite happy to receive (_|_) as an argument - so, hx and hy are well-defined.

thanks, max

instance (HTypeable a, HTypeable b) => HTypeable (Either a b) where toHType m = Defined "Either" [hx, hy] [ Constr "Left" [hx] [hx] {-Nothing-} , Constr "Right" [hy] [hy] {-Nothing-}] where (Left x) = m (Right y) = m hx = toHType x hy = toHType y

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