[sclv]

This is an interesting question. As much more of a Haskeller than an MLer I don't tend to feel the "need" to structure my programs explicitly modularly, and when I do I sort of instinctively use a combination of typeclasses and polymorphic higher-order functions to do so.

More basically, just think "everywhere I would open a module, instead I can take a parameterized record of functions" (and furthermore, if a datatype uniquely determines that record, I can associate a typeclass to that datatype). There are limitations to this, but in fewer circumstances than you'd think -- mainly about sort of cross-modularity (aka the expression problem).

There was a very nice discussion on lennart's blog about this in 2008, with a problem posed and some partial solutions (read bottom post to top):

http://augustss.blogspot.com/2008_12_01_archive.html

[z1mm32m4n]

This is actually quite a far way towards what I was looking for! Thank you very much.

> We have basically packaged up the dictionary and unpack it ourselves to get access to the operations. It's not pleasant, but it works.

Would you say this method of using multi-parameter type classes and packaging/unpackaging related operations in a record is "idiomatic"? I admittedly haven't browsed all that much Haskell code, but I feel like I don't see it used that frequently.

[sclv]

once you end up with a typeclass and associate the methods the unpacking goes away and you're just working in a context parameterized by some typeclass. i expressed it via that route to help make the connection to modules more clear. (there are occasions when you don't take that last step too, which is why i also sort of pointed towards that route). lennart's post shows an example where this sort of falls down -- but the followup also shows a nice haskelly solution that works, mainly, except when we want to intermix. he also suggests explicit type arguments as a way to make things nicer -- those have now landed in GHC :-)

the other relevant work in a broader sense that I should mention is regarding effectful contexts where idiomatically you declare a subclass of monad with the relevant operations, then instantiate it via the mtl or some other means, so you can swap out the IO backed "real" one or various harnesses or add in logging layers, etc.

finally, i guess i should add that as a rule of thumb i've noticed that purity and laziness both help provide ways to give "modular separation of concerns" directly. in particular, the most obvious thing we can do is just have each function do one thing to a bit of data, and produce a different bit of data and that's innately modular. but when we're interleaving IO (for example with mutable datastructures) and concerned with _when_ computation happens (in a strict setting), then it feels we're paying for this too much because we get big intermediate structures. but if you get the knack of just using pure lazy structures directly, you can sort of "amortize out" the computation cost in a nice way and also the space cost (as conceptually some big data structures become produced "on demand"). of course if you get it wrong, blammo :-)