Hello This is a formulation after more discussion on #haskell. This is for build-depends with the configurable things. Not sure whether this is the best way. - Einar Karttunen bexp = package-with-version | '(' bexp ')' | bexp '|' bexp | bexp ',' bexp | 'Flag(' string ')' | 'T' Semantics: package-with-version * satisfied iff a suitable version of P installed * depends on p '(' e ')' * satisfied iff e satisfied * dependencies from e a '|' b * satisfied iff a or b is satisfied * if a is satisfied then a else b a ',' b * satisfied iff a and b are satisfied * dependencies are the combination of a and b * union of packages and intersection of versions 'Flag(' string ')' * satisfied iff string in environment * no dependencies 'T' * always satisfied * no dependencies Code follows: data BExp = BP String Version | BOr BExp BExp | BAnd BExp BExp | BFlag String | BTrue eval (BP n v) = return [(n,v)] eval (BOr a b) = eval a `mplus` eval b eval (BAnd a b) = do x <- eval a y <- eval b return [(name, intersectVersions $ map snd xs) | xs@((name,_):_) <- groupByFst (x++y) ] liftM2 (comb) (eval a) (eval b) eval (BFlag s) = getFlag s >> return [] eval (BTrue) = return [] groupByFst = groupBy eqFst where eqFst (a,_) (b,_) = a == b - Einar Karttunen