22 Jun
2004
22 Jun
'04
7:38 a.m.
On Mon, Jun 21, 2004 at 06:03:21PM +0100, 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?
One way to do this would be to use record update syntax, but you would have to have the same field in all type-parameter-independent constructors. data E a b = L1 { r :: a } | L2 { r :: a } | L3 { r :: a } | L4 { r :: a } | R b deriving Show f g (R a) = R (g a) f _ other = other { r = r other } Best regards, Tom -- .signature: Too many levels of symbolic links