package preface

Preface

When learning functional programming, one is often confronted with constructs derived (or not) from category theory. Languages such as Haskell offer very complete libraries to use them, and thus, facilitate their learning. In OCaml, it often happens that these abstractions are buried in the heart of certain libraries/projects (Lwt, Cmdliner, Bonsai, Dune etc.). This is why one of the objectives of Preface is to propose tools for concretising these abstractions, at least as a pedagogical tool.

Is Preface useful

Since OCaml allows for efficient imperative programming, Preface is probably not really useful for building software. However, we (the maintainers) think that Preface can be useful for a few things:

• technical experimentation with abstractions (especially those from the Haskell world) that allow programming in a fun style.
• As an educational tool. Many teaching aids generally only offer the minimal interfaces to these abstractions. Preface tries to be as complete as possible.
• It was a lot of fun to make. The last point is obviously the lightest but building Preface was really fun! So even if some people won't see the point... we had fun making it!

Installation

The package is available on OPAM, opam install preface should be enough. (And by describing, of course, OPAM in your project OPAM file and linking it to your project in the standard way proposed by your build-system.)

OPAM pin

If you would like to use the latest version of Preface (under development) you can use the pin mechanism.

...
depends: [
  ...
  "preface" {pinned}
]

pin-depends: [
  ["preface.dev" "git+ssh://git@github.com/xvw/preface.git"]
  ...
]
...

Esy resolution

The library can also be installed with esy using a resolution in your package.json file :

...
    "dependencies": {
      ...
      "@opam/preface":"*"
    },
    "resolutions": {
        "@opam/preface":"xvw/preface#<commit>"
    },
...

The pattern of the resolution is xvw/preface#<commit> where <commit> is mandatory and should point to a specific commit.

Library anatomy

The library is divided into four parts (in the user area) which serve complementary purposes.

Available abstractions in Make and Specs

Although Stdlib offers common and, in our view, useful implementations, the heart of Preface lies in its ability to build the concretisation of abstractions for all sorts of data structures. Here is a list of abstractions that can be built relatively easily. As you can see, the diagram is heavily inspired by the Haskell community's Typeclassopedia.

Obviously, the set of useful abstractions is still far from being present in Preface. One can deplore the absence, for example, of contravariant Divisible functors, and probably many others. We have decided to privilege those for which we had a short and medium term use. But if you find that an abstraction is missing, the development of Preface is open, don't hesitate to contribute by adding what was missing.

Concretisation in Stdlib

As for the implemented abstractions, we favoured objects that we often manipulated (that we constantly reproduced in our projects) and also those that allowed us to test certain abstractions (Predicate and Contravariant for example). Don't hesitate to add some that would be useful for the greatest number of people!

Stdlib convention

As it is possible to take several paths to realise an abstraction, we decided to describe each abstraction in a dedicated sub-module. For example Option.Functor or Option.Monad to let the user choose which combinators to use.

Do not shadow the standard library

Although it was tempting to extend the standard OCaml library with this technique:

module Preface : sig
  module List : sig
    include module type of List
    include module type of Preface_stdlib.List
  end
end

We have decided not to do this to ensure consistent documentation (not varying according to the version of OCaml one is using).

Some design choices

Abstractions must respect a minimum interface, however, sometimes there are several paths to describe the abstraction. For example, building a monad on a type requires a return (or pure depending on the convention in practice) and:

• bind (>>=)
• map and join
• or possibly >=>

In addition, on the basis of these minimum combinators, it is possible to derive other combinators. However, it happens that these combinators are not implemented in an optimal way (this is the cost of abstraction). In the OCaml ecosystem, the use of polymorphic variants is sometimes used to give the user the freedom to implement, or not, a function by wrapping the function definition in a value of this type:

val f : [< `Derived | `Custom of ('a -> 'b)]

Instead of relying on this kind of (rather clever!) trick, we decided to rely mainly on the module language.

To make it easy to describe the embodiment of an abstraction, but still allow for the possibility of providing more efficient implementations (that propagate new implementations on aliases, such as infix operators, or functions that use these functions), Preface proposes a rather particular cut.

Each abstraction is broken down into several sub-modules:

The functors exposed in Make allow you to build each component one by one (Core, Operation, using Core, and Infix and Syntax using Core and Operation) and then group all these modules together to form the abstraction. Or use the Happy Path, which generally offers a similar approach to functors which builds Core but builds the whole abstraction.

Here is an example of the canonical flow of concretisation of an abstraction:

Although it is likely that the use of the Happy Path covers a very large part of the use cases and that it is not necessary to concretise every abstraction by hand, it is still possible to do so.

In addition, it is sometimes possible to describe one abstraction by specialising another. In general, these specialisations follow this naming convention: From_name (More_general_module) or To_name (Less_general_module) and sometimes you can build a module on top of another, for example Selective on top of Applicative and the naming follows this convention: Over_name (Req), ie Selective.Over_applicative

Projects using Preface

You use Preface for one of your projects and you want to be in this list? Don't hesitate to open a PR or fill an issue, we'd love to hear from you.

closing remarks

Preface is a fun project to develop and we have learned a lot from it. We hope you find it useful and/or enjoyable to use. We are open to any improvements and open to external contributions!

We received a lot of help during the development of Preface. Feel free to go to the CREDITS page to learn more.