There's work to provide this "module by contract" notion you're talking about exactly (http://www.mpi-sws.org/~skilpat/backpack/ http://www.mpi-sws.org/~skilpat

There's work to provide this "module by contract" notion you're talking about exactly (http://www.mpi-sws.org/~skilpat/backpack/ http://www.mpi-sws.org/~skilpat/backpack/) but it's very new and far from implementation. OCaml does a good job of this and is usually the poster child for parametric modules done right. That said, dynamic languages allow this kind of contractual module imputation by the virtue of allowing absolutely anything to be considered a module and hashed out at runtime. At the least this means carrying around a lot of tagging/runtime typing information so you can call up runtime errors. At the most, it means that your whole system is held together by an increasingly wide set of unchecked and uncheckable assumptions. Cabal hell is annoying but at least it's checking.