Update Modules and Functors Tutorials

[cuihtlauac]

Reorganise, update and complete tutorials on Modules and Functors. The draft outline follows. Top-level items are stand-alone tutorials.

• Introduction to the Module System
  • In “Get Started” or “Module System” (TBD)
  • Story on names & clashes
  • Separate Compilation
  • Separation of Concerns
  • Information Hidding / Encapsulation
  • Each item has min two links
    • Wikipedia or ext. ref
    • Relevant OCaml tutorial or ref. man
• Using Modules
  • Using Standard Library & Opam modules (content from 1st program)
• Writing Modules
  • Reference to “1st” program + Additional Material
• Module Includes and Hierarchy
  • Submodules (extracted from existing “Modules” doc)
  • Module Inclusion (extracted from existing “Modules” doc)
  • Dune include_subdirs qualified/unqualified
• Encapsulation
  • Module Signatures
  • Private definitions
  • Abstract Types (extracted from existing “Modules” doc)
  • Keyword private (Ref man chap 12)
• Using Functors
  • Signatures
  • Instantiating Functors
• Writing Functors
  • Defining a Functor

Open questions:

• Reference or duplicate stuff from “Get Started”
  • Modules and Standard Library in “A Tour of OCaml”
  • Your First OCaml Program
• Relationship with Set and Map tutorials
• PR Import Rewrote Set Tutorial from V2 PR #948

P.S. As of creating this PR, it doesn't patch anything. Only discussion

[leostera]

I think the older description was better here since it describes a functor by analogy with something else that people may be familiar with.

I'd take it a little further and say that functors are functions but that instead of regular values they take modules as inputs and create modules as outputs.

This intuition I think can help with other things, like the "transitive closures leading to modules" prerequisite.

[cuihtlauac]

Sorry, I should have said that what I've written here is interim text. The original intro had some good ideas but it needs to be rewritten. This also applies to the transitive closure thing, of course we're not going to publish this as is.

[leostera]

No worries, and nothing to be sorry for! 🙏🏼

[cuihtlauac]

Should we also say something like this?

The term functor comes from a branch of mathematics called category theory. Although OCaml functors are loosely inspired by them, no knowledge of that math is required to understand and work with OCaml functors. Functors in Haskell more closely represent the Category Theory concept and are, have nothing in common with OCaml functors.

[christinerose]

Some initial comments/suggestions/questions
Haven't double checked the code yet.

[christinerose]

Initial comments/suggestions/questions for functors.md