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: tom.schrijvers@cs.kuleuven.be Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm