Organize for minimal imports.

[sellout]

To make it simple for clients to use the library (and to try to encourage a particular usage pattern), we should organize it so a user almost always needs only a single import per file. Here are the three use cases I see for different parts of a program:

when defining algebras

• algebra types
• Recursive/Corecursive/FunctorT/TraverseT types
• project/embed/map/traverse

when using algebras

• AlgebraOps
• folds/unfolds/refolds
• Possibly rec/Corecursive/etc instances
• Distributive laws

when creating recursive data

• fixpoint types & their rec/Corecursive/... Instances

For each of these cases, there should be two imports – one that provides everything via “ops” and one that provides package methods.

[sellout]

There is a fourth category – when defining pattern functors. This sometimes overlaps with the first category (you define algebras in the same file you define your pattern functors), but often doesn’t. I don’t think there’s much in it, but at least Delay, so instances for Delay[Equal, F], etc. can be defined in the functor’s companion object.