Module Bare_sigs.Monad

Lwt, result, and Lwt-result monad operators

This module provides the necessary functions and operators to use Lwt, result and Lwt-result as a monad.

Basics

The three, tiered monads have each their full set of operators:

The Lwt monad:

• Lwt.return for return,
• Lwt.bind or (>>=) for bind, and
• Lwt.map or (>|=) for map.

The result monad:

• Result.ok or ok for return,
• Result.bind or (>>?) for bind, and
• Result.map (>|?) for map.

In addition, Result.error or error is for failures within the result monad.

The Lwt-result monad:

• return or Lwt.return_ok for return,
• (>>=?) for bind, and
• (>|=?) for map.

In addition, fail is for the failure within the Lwt-result combined monad.

Note that future improvements are planned to (a) make those more uniform, (b) allow the opening of selected infix operators only, and (c) provide let*-binds.

Preallocated values

The module also provides preallocated values for the common types:

• unit_s (resp unit_e) (resp unit_es) is Lwt.return () (resp Ok ()) (resp Lwt.return (Ok ())),
• none_s (resp none_e) (resp none_es) is Lwt.return None (resp Ok None) (resp Lwt.return (Ok None)),
• etc. (see full inventory below)

Note that some of these values are also available in their dedicated modules. E.g., none_* are available in Option.

Joins

The join_p function takes a list of promises ps and returns a single promise p that resolves with () when all the promises of ps have resolved.

The all_p function takes a list of promises ps and returns a single promise p that resolves when all the promises of ps have resolved. The value p resolves to is the list of values the promises of ps resolve to. The order is preserved.

The both_p function takes two promises p1 and p2 and returns a single promise p that resolves when both promises p1 and p2 have resolved. The value p resolves to is the tuple of values the promises p1 and p2 resolve to.

Note that like all _p functions, these functions have a best-effort semantic: they only resolve once all the underlying promises have resolved.

The _e variants are equivalent for the result monad: the final result is Ok if all the underlying results are Ok.

The _es variants are equivalent for the Lwt-result monad: the final promise resolves to Ok if all the underlying promise resolve to Ok.