Chris Kuklewicz wrote:
This is how I would write getLeaves, based on your GADT:
data IsLeaf
data IsBranch
newtype Node = Node { getNode :: (forall c. ANode c) }
data ANode :: * -> * where
Branch :: String -> String -> (ANode a,ANode b) -> [Node] ->
ANode IsBranch
Leaf :: String -> String -> ANode IsLeaf
getLeaves :: ANode a -> [ANode IsLeaf]
getLeaves (Branch _ _ (l1,l2) rest) = getLeaves l1 ++ getLeaves l2
++ concatMap (getLeaves.getNode) rest
getLeaves x@(Leaf {}) = [x]
Thanks Chris - that's really neat!
I see it's the explicit wrapping and unwrapping of the existential that
solves the typechecking problem, and the use of newtype ensures there's no
run-time penalty for this.
Also the wrapping of the existential allowed higher order functions to be
used making the code much neater.
Regarding the question of why in the original example the typechecker was
trying to match (forall b.ANode b) against (ANode a) and not (ANode IsLeaf),
I think the answer is probably that the typechecker first finds the MGU of
the types occupying the same position in all the left hand sides first, then
it tries to match this against the declared type at that position, whereas
for the original example to have typechecked it would have to treat each
equation separately. Anyway it's now an irrelevant point given the clarity
of your solution which compiles fine,
Best regards, Brian.
--
http://www.metamilk.com
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