Hello everyone, I've been banging my head against my desk a bit so I figured it's time to ask for help :-) I'm writing an application that persists data to disk. The hard stuff is pretty much done (binary serialisation, etc...) The big stumbling block is that I want users to be able to define their own schemas. This means that the keys for a data store may be Integer, Strings, etc... I have a BTreeIndex type: data BTreeIndex a b = Branch { parentBT :: !BlockPtr, keysBT :: ![a], kidsBT :: ![BlockPtr] } | Leaf { parentBT :: !BlockPtr, valsBT :: !(LeafMap a b), prevBT :: !PrevLeaf, nextBT :: !NextLeaf } type IdxPS = BTreeIndex PackedString BlockPtr type IdxInt = BTreeIndex Integer BlockPtr When a user queries I have to read the input from IO and then somehow cast the key/index type without angering the type checker. If I omit the following function I get the ominous "Ambiguous type variable a..." error. If I use it I get a complaint that the checker was expecting an Integer rather than a PackedString. coerceIndex f (Schema _ SInt SPtr _) (r,hdl,o,hdr) = f (r::IdxInt,hdl,o,hdr) coerceIndex f (Schema _ SStr SPtr _) (r,hdl,o,hdr) = f (r::IdxPS,hdl,o,hdr) Where f is a curried select function (e.g. selectAll, selectByKey, etc...) waiting for the final tuple containing the index and other miscellanea that is needed to read the data. I'm hopelessly lost and I assume that the many brilliant minds on this list will chuckle, pat me on the head and tell me what I've been doing wrong. Thanks for your time guys, Scott