[clean-list] generic equality?

[Pieter Koopman]

Hi Jerzy,

At 12:01 25-10-2004 +0200, Jerzy Karczmarczuk wrote:
>Forgive me a simplistic question.
>Since in Clean there is no 'deriving' contraption as in Haskell, the
>definition of (simple, identity) equality for instances of the data type
>
>:: Dat = A | B | C | D | E | F | G | H | I | J | K | L | M | N | O
>//... etc.
>
>is the complete set of:
>(==) A A = True
>...
>(==) K K = True
>...
>(==) _ _  = False
>
>which is heavy and inelegant. What are the ways to do it better? The manual
>shows the example of generic constructs, e.g. for lists, which demand 
>first the
>conversion of an algebric datatype into sums/products which use Either, etc.
>But with many alternatives the conversion function is also ugly. Do you have
>any other ideas?

Use the predefined generic equality:

module ...

import StdEnv, StdGeneric, GenEq

:: Dat = A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P

derive gEq Dat

Start = P === K

If you prefer to use == instead of the generic version === you should add:

instance == Dat where (==) x y = x === y

Start = P == K

Ha fun,

Pieter Koopman