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Functional programming library for OCaml
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Clarity - functional programming library for OCaml


The goal of this project is to make pure functional programming idioms as useful as possible given OCaml's absence of higher-kinded types and typeclasses.

Main features are:

  • Standard "classes" like Functor-Applicative-Monad
  • Concrete instances like Reader-Writer-State
  • Useful data types like Either, These or Vector

Design notes

  • All concrete datatypes also have its constructors defined as values where name is prefixed with underscore. Sometimes it's more convenient to use "curried", first-class version of a constructor, e.g. following two are equivalent:
let long  = (fun x -> Some x) a
let short = _Some x
  • Applicative operator ap and its infix version (<~>) are "lazy" by its second argument. This allows for an applicative to "fail-fast" and don't compute unneeded values. "Strict" versions are called ap' and (<*>) respectively. "Laziness" here is just (unit -> 'a) closure, so you can use function combinators from Fn module for convenience:
open Clarity
open Option

val (<*>) : ('a -> 'b) t -> 'a t -> 'b t
val (<~>) : ('a -> 'b) t -> (unit -> 'a t) -> 'b t

val serialize : int -> int -> string -> string
val idx : int option
val long_computation : int -> int option
val title : string option

open Fn

let res : string Option.t =
  map serialize idx
    <~> defer long_computation 1024
    <*> title
  • Right folds are also "lazy" by "accumulator" argument of a folding function. Strict right fold is called foldr'. This allows for shortcut when function no more needs data. For example, here is any function from Foldable module that checks if at least one element of a Foldable satisfies given predicate:
let any p = foldr (fun x a -> p x || a ()) (const false)


You can find ocamldoc here.

Manual installation

$ make && make install
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