On 5/31/07, Stefan Holdermans <[EMAIL PROTECTED]> wrote:
Dan,
> If you want to enforce associativity just create your own Eq
> instance and
> make it a pattern there.
Could you elaborate on that? It's still early here and I've had only
one cup of of coffee yet.
Cheers,
Stefan
QuickCheck allows you to approximately verify properties by testing
them on randomly generated input. The stated properties thus cannot
formally be proved, but they act as a pretty good formal
specification. (Full automatic theorem proving for a language as
expressive as Haskell is impossible or infeasible. So we have to
approximate.)
prop_assocJoin x y z = join x (join y z) == join (join x y) z
-- or, more generally
associative :: (Eq a) => (a -> a -> a) -> a -> a -> a -> Bool
associative f x y z = f x (f y z) == f (f x y) z
prop_assocJoin = associative join
-- to check this for a given implementation of "join", you need to:
import Test.QuickCheck
Main> quickCheck prop_assocJoin
This also requires that QC can generate arbitrary values of type "e".
See the QuickCheck documentation for more infos on that:
http://www.cs.chalmers.se/~rjmh/QuickCheck/manual.html
/ Thomas
--
"Remember! Everytime you say 'Web 2.0' God kills a startup!" -
userfriendly.org, Jul 31, 2006
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