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# Play with Error-Monads in OCaml

PVEM actually stands for “Polymorphic Variants-based Error Monads.”

## Usage

The basic idea of these modules is to manipulate:

type ('ok, 'error) result = [ | `Ok of 'ok | `Error of 'error ]

or combinations of that type (why polymorphic variants? see

some implementation notes).

When you `open Pvem`

(or `include`

) in your source, you get

- the module
`Result`

which implements the interface`ERROR_MONAD`

with the above type, - the functor
`With_deferred`

:`DEFERRED`

→`DEFERRED_RESULT`

which

builds “error-monads” out of`Lwt`

-like

concurrency monads.

Since the monad(s) become pervasive; the

`ERROR_MONAD`

module type defines 3 infix operators:

`>>=`

(`ERROR_MONAD.bind`

): the “standard” bind operator.`>>|`

(`ERROR_MONAD.map`

): pass the value through a “pure” function.`>><`

(`ERROR_MONAD.destruct`

): “reopen” the error monad to be able to match

on both the “Ok” and “Error” cases; then return inside the monad like with

`bind`

.

## Examples

### Infix Operators

Here is a basic usage example for the 3 infix operators:

open Pvem open Result let fail_on_42 : int -> (int, string) Result.t = function 42 -> fail "42" | other -> return other let f_uses_bind_and_return: int -> (float, string) Result.t = fun x -> fail_on_42 x >>= fun not_42 -> return (2. *. float not_42) let f_uses_map: int -> (int * int, string) Result.t = fun x -> fail_on_42 x >>| (fun x -> (x, x)) let f_uses_destruct: int -> (float, string * string) Result.t = fun x -> fail_on_42 x >>< function | `Ok o -> return (float o) | `Error s -> fail ("did not convert to float", s)

### Functor

To get an error monad on top of Lwt, simply

module Deferred_result = Pvem.With_deferred(Lwt)

With Jane Street's Async `Deferred.t`

:

open Core_kernel.Std open Async.Std module Deferred_result = Pvem.With_deferred(struct include Deferred let catch tri wiz = Async.Std.try_with tri >>= function | Ok o -> return o | Error e -> wiz e end)