package seqes

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Seq with monads


Dune Dependency






Variations of the Seq module with monads folded into the type


monad seq

Published: 04 Jan 2023



A library to compose the abstraction of the Stdlib.Seq module with monads.

The Stdlib.Seq module doesn't allow mixing in monads. Specifically, the Stdlib.Seq module provides the following type definition:

type 'a t = unit -> 'a node
and +'a node =
  | Nil
  | Cons of 'a * 'a t

in which an 'a Seq.t must be a simple lambda returning a node. You cannot thread that lambda into a monad.

The Seqes library provides functors to generate new Seq-like modules, adapted to your monad.


The functor Seqes.Standard.Make1 takes a monad parameter.

module SeqM = Seqes.Standard.Make1(Monad)

It returns a module containing traversors using the provided Monad.

SeqM.iter : ('a -> unit Monad.t) -> 'a Stdlib.Seq.t -> unit Monad.t

Check examples/seqlwt/ for an example use of Seqes.Standard.Make1.


The functor Seqes.Monadic.Make1 takes a monad parameter.

module SeqM = Seqes.Monadic.Make1(Monad)

It returns a module with a fresh type definition integrating the Seq abstraction and the monad together:

type 'a t = unit -> 'a node Monad.t
and 'a node =
  | Nil
  | Cons of 'a * 'a t

It also exports all the functions from the Stdlib.Seq module but operating on that new type instead.

SeqM.iter : ('a -> unit) -> 'a SeqM.t -> unit Monad.t

The library provides more advance usage. Specifically, note how the SeqM.iter function takes a function 'a -> unit, and how it is sometimes useful to pass a 'a -> unit Monad.t function instead.

The module produce by Seqes.Monadic.Make1 actually contains a submodule M with those specialised variants.

SeqM.M.iter : ('a -> unit Monad.t) -> 'a SeqM.t -> unit Monad.t

In addition to this submodule M, the module contains two functors to generate more specialised sets of functions. See examples/seqlist/ for an example of this even more advanced use.

Seqes.Standard.Make2 and Seqes.Monadic.Make2

If your monad has two type parameters (e.g., ('a, 'e) result), then you can use Seqes.Standard.Make2 and Seqes.Monadic.Make2 instead of the functors mentioned above.

See examples/seqres/ and examples/seqlwtres/ for examples.

Dependencies (2)

  1. dune >= "3.0"
  2. ocaml >= "4.14"

Dev Dependencies (4)

  1. odoc with-doc
  2. alcotest with-test & >= "1.5.0"
  3. qcheck-alcotest with-test
  4. qcheck with-test & >= "0.19"




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