[Haskell-cafe] A question about constraints

[Bulat Ziganshin]

Hello jean-christophe,

Thursday, October 2, 2008, 1:46:20 PM, you wrote:

> If one wants to use pattern matching,

afaik we had so-called views in early haskell versions. they proivide
way to define two-way constructors - used for deconstruction via
pattern-matching too

views wa removed from haskell (or not included) in 1998 because they
makes harder some theoretic operation, i don't remeber exactly

finally, ghc now includes some form of active patterns which may be
used to define your own way to decompose values. but their syntax
isn't compatible with constructors so you can't define complex type
which mimicks simple ones, and in particular you can replace simple
type with complex one w/o rewriting all the client code

imho it's serious lack in haskell support for Abstract Data Types


-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com