David Roundy wrote:
class Commutable a b d c
commute :: Commutable a b d c =>
(Patch a b, Patch b c) -> (Patch a d, Patch d c)
But for this to work properly, I'd need to guarantee that
1. if (Commutable a b d c) then (Commutable a d b c)
2. for a given three types (a b c) there exists at most one type d
such that (Commutable a b c d)
The problem seems easily solvable, exactly as described. We need to
take care of the two requirements separately.
{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-undecidable-instances #-}
module D1 where
data Patch a b = Patch String deriving Show
-- This is identical to what Tom Schrijvers wrote
class Commutable a b c d |
a b c -> d, -- 2.
a d c -> b -- based on 1. + 2.
-- But how do we make sure that Commutable a d c b exists whenever
-- Commutable a b c d does? very easily: with the help of another
-- type class
instance (Commutable' a b c d, Commutable' a d c b)
=> Commutable a b c d
I had not thought of using a double constraint. Very clever.
Why not also put this constraint in the class declaration? You don't want
any other instances of Commutable (or do you?):
class (Commutable' a b c d, Commutable' a d c b)
=> Commutable a b c d |
a b c -> d, -- 2.
a d c -> b -- based on 1. + 2.
Cheers,
Tom
--
Tom Schrijvers
Department of Computer Science
K.U. Leuven
Celestijnenlaan 200A
B-3001 Heverlee
Belgium
tel: +32 16 327544
e-mail: [EMAIL PROTECTED]
Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm
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