package fmlib_std

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Overview
API
Monad

package fmlib_std

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Result: Handling results of operations which can fail

Overview

Operations returning a result type can be used to have some functional exception handling.

Let's say that you have some operatios returning a result object.

let op1 ...  : (int,    error) result  = ...
let op2 ...  : (char,   error) result = ...
let op3 ...  : (string, error) result = ...
let op3 ...  : (t,      error) result = ...

You can chain these operations by concentrating on the success case only and handling the error case at the end of the chain.

match
    let* i = op1 ...  in
    let* c = op2 ... i ...  in
    let* s = op3 ... i ... c ...  in
    op4 ... i ... c ... s
with
| Ok x ->
    (* Handling of the success case *)
| Error e ->
    (* Handling of the error case which might have
       occurred in any of the steps *)

A simple example:

type 'a r = ('a, string) result

let add (a: int r) (b: int r): int r =
    let* x = a in
    let* y = b in
    Ok (x + y)

let divide (a: int r) (b: int r): int r =
    let* x = a in
    let* y = b in
    if y = 0 then
        Error "Division by Zero"
    else
        Ok (x / y)

assert (
    add (Ok 1) (divide (Ok 2) (Ok 0))
    =
    Error "Division by Zero"
)

assert (
    add (Ok 1) (divide (Ok 10) (Ok 2))
    =
    Ok 6
)

API

type ('a, 'e) t = ('a, 'e) result

'a is the result type in case of success and 'e is the result type in case of failure. It is implemented by the ocaml type result from the ocaml standard library.

val return : 'a -> ('a, 'e) t

return a Equivalent to Ok a.

val fail : 'e -> ('a, 'e) t

fail e Equivalent to Error e.

val to_option : ('a, 'e) t -> 'a option

to_option r Map r to an optional element i.e. Some a in case of Ok a and None in case of Error _.

val (>>=) : ('a, 'e) t -> ('a -> ('b, 'e) t) -> ('b, 'e) t

m >>= f

maps success result m to f a. In case of an error result f is not called and the error remains.

val let* : ('a, 'e) t -> ('a -> ('b, 'e) t) -> ('b, 'e) t

let* a = m in f a is the same as m >>= f

val map : ('a -> 'b) -> ('a, 'e) t -> ('b, 'e) t

map f m Map the result in m via the function f.

val map_error : ('e -> 'f) -> ('a, 'e) t -> ('a, 'f) t

map_error f m Map the error in m via the function f.

val get : ('a, Void.t) t -> 'a

get m Get the ok content of a result object which cannot have errors.

Monad

module Monad (E : Interfaces.ANY) : sig ... end

The result type encapsulated in a module which satisfies the monadic interface.