package containers

  1. Overview
  2. Docs
package containers
Legend:
Library
Module
Module type
Parameter
Class
Class type

Error Monad

Uses the new "result" type from OCaml 4.03.

  • since 0.16
type 'a sequence = ('a -> unit) -> unit
type 'a equal = 'a -> 'a -> bool
type 'a ord = 'a -> 'a -> int
type 'a printer = Buffer.t -> 'a -> unit
type 'a formatter = Format.formatter -> 'a -> unit

Basics

type (+'good, +'bad) t = ('good, 'bad) Result.result =
  1. | Ok of 'good
  2. | Error of 'bad
val return : 'a -> ('a, 'err) t

Successfully return a value

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

Fail with an error

val of_exn : exn -> ('a, string) t

of_exn e uses Printexc to print the exception as a string

val of_exn_trace : exn -> ('a, string) t

of_exn_trace e is similar to of_exn e, but it adds the stacktrace to the error message.

Remember to call Printexc.record_backtrace true and compile with the debug flag for this to work.

val fail_printf : ('a, Buffer.t, unit, ('a, string) t) format4 -> 'a

fail_printf format uses format to obtain an error message and then returns Error msg

val fail_fprintf : ('a, Format.formatter, unit, ('a, string) t) format4 -> 'a

fail_printf format uses format to obtain an error message and then returns Error msg

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

Map on success

val map_err : ('err1 -> 'err2) -> ('a, 'err1) t -> ('a, 'err2) t

Map on the error variant

val map2 : ('a -> 'b) -> ('err1 -> 'err2) -> ('a, 'err1) t -> ('b, 'err2) t

Same as map, but also with a function that can transform the error message in case of failure

val iter : ('a -> unit) -> ('a, _) t -> unit

Apply the function only in case of Ok

exception Get_error
val get_exn : ('a, _) t -> 'a

Extract the value x from Ok x, fails otherwise. You should be careful with this function, and favor other combinators whenever possible.

val get_or : ('a, _) t -> default:'a -> 'a

get_or e ~default returns x if e = Ok x, default otherwise

val map_or : ('a -> 'b) -> ('a, 'b) t -> default:'b -> 'b

map_or f e ~default returns f x if e = Ok x, default otherwise

val catch : ('a, 'err) t -> ok:('a -> 'b) -> err:('err -> 'b) -> 'b

catch e ~ok ~err calls either ok or err depending on the value of e.

val flat_map : ('a -> ('b, 'err) t) -> ('a, 'err) t -> ('b, 'err) t
val (>|=) : ('a, 'err) t -> ('a -> 'b) -> ('b, 'err) t
val (>>=) : ('a, 'err) t -> ('a -> ('b, 'err) t) -> ('b, 'err) t
val equal : ?err:'err equal -> 'a equal -> ('a, 'err) t equal
val compare : ?err:'err ord -> 'a ord -> ('a, 'err) t ord
val fold : ok:('a -> 'b) -> error:('err -> 'b) -> ('a, 'err) t -> 'b

fold ~ok ~error e opens e and, if e = Ok x, returns ok x, otherwise e = Error s and it returns error s.

Wrappers

val guard : (unit -> 'a) -> ('a, exn) t

guard f runs f () and returns its result wrapped in Ok. If f () raises some exception e, then it fails with Error e

val guard_str : (unit -> 'a) -> ('a, string) t

Same as guard but uses of_exn to print the exception.

val guard_str_trace : (unit -> 'a) -> ('a, string) t

Same as guard_str but uses of_exn_trace instead of of_exn so that the stack trace is printed.

val wrap1 : ('a -> 'b) -> 'a -> ('b, exn) t

Same as guard but gives the function one argument.

val wrap2 : ('a -> 'b -> 'c) -> 'a -> 'b -> ('c, exn) t

Same as guard but gives the function two arguments.

val wrap3 : ('a -> 'b -> 'c -> 'd) -> 'a -> 'b -> 'c -> ('d, exn) t

Same as guard but gives the function three arguments.

Applicative

val pure : 'a -> ('a, 'err) t

Synonym of return

val (<*>) : ('a -> 'b, 'err) t -> ('a, 'err) t -> ('b, 'err) t

a <*> b evaluates a and b, and, in case of success, returns Ok (a b). Otherwise, it fails, and the error of a is chosen over the error of b if both fail.

val join : (('a, 'err) t, 'err) t -> ('a, 'err) t

join t, in case of success, returns Ok o from Ok (Ok o). Otherwise, it fails with Error e where e is the unwrapped error of t.

val both : ('a, 'err) t -> ('b, 'err) t -> ('a * 'b, 'err) t

both a b, in case of success, returns Ok (o, o') with the ok values of a and b. Otherwise, it fails, and the error of a is chosen over the error of b if both fail.

Infix

module Infix : sig ... end

Collections

val map_l : ('a -> ('b, 'err) t) -> 'a list -> ('b list, 'err) t
val fold_l : ('b -> 'a -> ('b, 'err) t) -> 'b -> 'a list -> ('b, 'err) t
val fold_seq : ('b -> 'a -> ('b, 'err) t) -> 'b -> 'a sequence -> ('b, 'err) t

Misc

val choose : ('a, 'err) t list -> ('a, 'err list) t

choose l selects a member of l that is a Ok _ value, or returns Error l otherwise, where l is the list of errors.

val retry : int -> (unit -> ('a, 'err) t) -> ('a, 'err list) t

retry n f calls f at most n times, returning the first result of f () that doesn't fail. If f fails n times, retry n f fails with the list of successive errors.

module type MONAD = sig ... end
module Traverse (M : MONAD) : sig ... end

Conversions

val to_opt : ('a, _) t -> 'a option
val of_opt : 'a option -> ('a, string) t
val to_seq : ('a, _) t -> 'a sequence

IO

val pp : 'a printer -> ('a, string) t printer
val print : 'a formatter -> ('a, string) t formatter
OCaml

Innovation. Community. Security.

GitHub Discord Twitter Peertube RSS
About
Changelog Releases Industrial Users Academic Users Why OCaml
Ecosystem
Packages Community Events OCaml Planet Jobs
Resources
Install OCaml Get Started Platform Tools Language Manual Standard Library API Books Exercises Papers OCaml Playground Logo
Policies
Carbon Footprint Governance Privacy Code of Conduct