[Haskell-cafe] My Unification Sorrows
stefanor at cox.net
Wed Apr 4 22:21:45 EDT 2007
On Wed, Apr 04, 2007 at 07:16:35PM -0700, Paul Berg wrote:
> I believe I may have found a solution (I *think* it's correct):
> The occurs check needs to stay, but be modified for infinite types.
> When the occurs check is true, instead of failing, we should keep the
> constraint, but skip performing substitution on the rest of the list
> of constraints. This allows us to not fall into the trap of an
> infinite substitution loop. So, unify becomes, in the case of
> infinite types:
> -- | Given a list of constraints, unify those constraints,
> -- finding values for the type variables
> unify :: (Monad m, Functor m) => [Constraint] -> m [Constraint]
> unify  = return 
> unify ((t1 , t2) :rest)
> | t1 == t2 = unify rest
> unify ((tyS, tyX@(TVar _)) :rest)
> -- | tyX `occursIn` tyS = fail "Infinite Type"
> | tyX `occursIn` tyS = fmap ((tyX,tyS):) (unify rest)
> | otherwise = fmap ((tyX,tyS):) (unify
> $ substinconstr tyX tyS rest)
> unify ((tyX@(TVar _), tyT) :rest) = unify $ (tyT,tyX) : rest
> unify ((tyS1 :-> tyS2,tyT1 :-> tyT2):rest) = unify $ (tyS1,tyT1) :
> (tyS2,tyT2) : rest
> unify _ = fail "Unsolvable"
I honestly don't understand the algorithm. (And I don't have tapl!!)
So, ... quickcheck! You've seen my occurs-free unifier; now write an
Arbitrary instance for trees and check that they always give the same
result. "The chance of a common bug is neglible.".
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