Well, I can live with it well enough, but I wanted to make sure I wasn't overlooking something simple (which it appears I'm not). #g -- At 13:38 22/06/04 +0100, Adrian Hey wrote:
I think this was the topic of my very first post to Haskell mailing list, many years ago..
http://www.dcs.gla.ac.uk/mail-www/haskell/msg00452.html
I think the answer is no. Apparently this is feature (I still think it's a bug though:-)
Regards -- Adrian Hey
On Monday 21 Jun 2004 6:03 pm, Graham Klyne wrote:
If I have a polymorphic algebraic type (T a) with several type constructors, only one of which actually references the type parameter, is there any way to express type conversion for the type-parameter-independent constructors without actually mentioning all the constructors?
Here's a simple example based on Either:
[[ data A = A String deriving (Show, Eq) data B = B String deriving (Show, Eq)
f :: (a->b) -> Either String a -> Either String b f g (Right a) = (Right $ g a) f g (Left s) = (Left s) -- f g (s) = (s) -- doesn't work
a2b (A s) = (B s)
t1 = f a2b (Left "x") t2 = f a2b (Right (A "y")) ]]
The second case for 'f' throws a type error when the constructor 'Left' is omitted, because the type of 's' is fixed to be Either String A when the required result (in this case, because of a2b) is Either String B.
#g
------------ Graham Klyne For email: http://www.ninebynine.org/#Contact
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------------ Graham Klyne For email: http://www.ninebynine.org/#Contact