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#toHType
) 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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