package preface
Install
Dune Dependency
Authors
Maintainers
Sources
sha256=c29e7db45487aae9328e365f6fd93c9ad0af21f507a918c54d7d8313f1e12259
sha512=2e6cde461dcfc07d7e6764398d336ceb5d9baf22672fc00bfbf07d31607defa55323a7d33696fbabea59c0e00c8dfa603ae143e277939a059c9c3bf08d51ba64
Description
Preface is an opinionated library designed to facilitate the handling of recurring functional programming idioms in OCaml.
README
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 buildsystem.)
OPAM pin
If you would like to use the latest version of Preface (under development) you can use the pin mechanism.
...
depends: [
...
"preface" {pinned}
]
pindepends: [
["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.
Library  Description 

preface.specs 
Contains all the interfaces of the available abstractions. The specifications resemble the _intf suffixed signatures found in other libraries in the OCaml ecosystem. 
preface.make 
Contains the set of functors (in the ML sense of the term) for concretising abstractions. Schematically, a module in Preface.Make takes a module (or modules) respecting a signature described in Preface.Specs to produce a complete signature (also described in Preface.Specs ). 
preface.stdlib 
Contains concrete implementations, constructs that implement abstractions described in Preface.Specs by means of the functors present in Preface.Make . This library is, at least, an example of the use of Specs and Make . 
preface 
Packs all libraries making Preface.Specs and Preface.Make accessible as soon as Preface is available in the current scope. And includes Preface.Stdlib (so everything in Preface.Stdlib is available from Preface). 
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!
Name  Description  Abstractions 

Approximation.Over 
A generalization of Const (the phantom monoid) for over approximation 
Applicative , Selective 
Approximation.Under 
Same of Over but for under approximation 
Applicative , Selective 
Continuation 
A continuation that can't be delimited  Functor , Applicative , Monad 
Env 
The env comonad using Identity as inner monad 
Functor , Comonad 
Either 
Represent a disjunction between left and right 
Bifunctor and can be specialised for the left part; Functor , Alt , Applicative , Monad , Traversable through Applicative and Monad, Foldable 
Fun 
Function 'a > 'b 
Profunctor , Strong , Choice , Closed , Category , Arrow , Arrow_choice , Arrow_apply 
Identity 
A trivial type constructor, type 'a t = 'a 
Functor , Applicative , Selective , Monad , Comonad 
List 
The standard list of OCaml  Foldable , Functor , Applicative , Alternative , Selective , Monad , Monad_plus , Traversable through Applicative or Monad, Monoid (where the inner type must be fixed) 
Nonempty_list 
A list with, at least, one element  Foldable , Functor , Alt , Applicative , Selective , Monad , Comonad , Traversable through Applicative or Monad, Semigroup (where the inner type must be fixed) 
Option 
Deal with absence of values  Foldable , Functor , Applicative , Alternative , Monad , Monad_plus , Traversable through Applicative of Monad, Monoid (where the inner type must be fixed) 
Predicate 
A generalization of function 'a > bool 
Contravariant , Divisible 
Reader 
The reader monad using Identity as inner monad 
Functor , Applicative , Monad 
Result 
Deal with Ok or Error values 
Bifunctor and can be specialised for the error part; Functor , Alt , Applicative , Monad , Traversable through Applicative and Monad, Foldable 
State 
The state monad using Identity as inner monad 
Functor , Applicative , Monad 
Store 
The store comonad using Identity as inner monad 
Functor , Comonad 
Stream 
Infinite list  Functor , Applicative , Monad , Comonad 
Traced 
The traced comonad using Identity as inner monad 
Functor , Comonad 
Try 
A biased version of Result with exception as the error part 
Functor , Alt , Applicative , Monad , Traversable through Applicative and Monad, Foldable 
Pair 
A pair 'a * 'b 
Bifunctor 
Validate 
A biased version of Validation with exception Nonempty_list as invalid part 
Functor , Alt , Applicative , Selective , Monad , Traversable through Applicative and Monad, Foldable 
Validation 
Like Result but the invalid part is a Semigroup for accumulating errors 
Bifunctor and can be specialized on the invalid part: Functor , Alt , Applicative , Selective , Monad , Traversable through Applicative and Monad, Foldable 
Writer 
The writer monad using Identity as inner monad 
Functor , Applicative , Monad 
Stdlib convention
As it is possible to take several paths to realise an abstraction, we decided to describe each abstraction in a dedicated submodule. 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
andjoin
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 submodules:
Submodule  Role 

Core 
This module describes all the fundamental operations. For example, for a monad, we would find return , map , bind , join and compose_left_to_right 
Operation 
The module contains the set of operations that can be described using the Core functions. 
Infix 
The module contains infix operators built on top of the Core and Operation . 
Syntax 
The module contains the let operators (such as let* and let+ for example), built with the Core and Operation functions. 
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
Project name  Description  Links 

YOCaml  YOCaml is a static blog generator that essentially takes advantage of Preface's Freer , Result , Validation and Arrow . 
Github repository 
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.
Dev Dependencies (5)

odoc
withdoc

mdx
withtest & < "2.0"

qcheckalcotest
withtest

qcheckcore
withtest & >= "0.11" & < "0.18"

alcotest
withtest
Used by
None
Conflicts
None