Hi all, I'm building a tree-like structure that somewhat resembles a package/ dependency structure as most packagemanagers/ports-systems have them. It's tree-like (Data.Tree at the moment), but I will probably need to make some structural changes to allow for more complex stuff like circular dependencies and stuff like 'alternatives'. While I'm still on the Tree (nice & simple), I want to explore some business-rules & validation stuff. This is a bit of a contrived example: Let's say that nodes are (NodeType, Int) pairs, where the Nodetype is just a simple algebraïc type. Now certain types of nodes are only allowed to 'grow' on top of some other nodes, but up to 10 levels deep. So certain nodes are only allowed if one of 10 ancestors is of some other type. Or to make this a bit weirder (maybe too contrived): its parent should have a 'brother' of some type. Ofcourse I can make a 'validate' function that checks if a tree plays by the rules, but I would like to put the typesystem to use somehow, since there aren't that many rules. So I would like to have an error thrown any time an invalid structure is used/created instead of having to validate myself every time. I would like this, because in the future I will need to 'morph' trees into each other so inbetween steps that exist must still be valid. First I thought about creating some auto-validate monad that just validates the result after every step, but before going that way I would like to make sure this is the way to go. I understand that Data.Tree (and other functors) just take 1 type 'payload' and they don't mind what's in there, while my 'validation' rules are about the payload _and_ the place in the tree, so I will probably have to change a bit here. Is this at all possible? Is it ok to do or is a 'external' validation/validation monad clearer? What would be the way to go? Thanks for any help! Mathijs