package containers

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Error Monad

Uses the new "result" type from OCaml 4.03.

  • since 0.16
type 'a iter = ('a -> unit) -> unit

Fast internal iterator.

  • since 2.8
type 'a equal = 'a -> 'a -> bool
type 'a ord = 'a -> 'a -> int
type 'a printer = Stdlib.Format.formatter -> 'a -> unit

Basics

type nonrec (+'good, +'bad) result = ('good, 'bad) Stdlib.result =
  1. | Ok of 'good
  2. | Error of 'bad
type (+'good, +'bad) t = ('good, 'bad) 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, Stdlib.Buffer.t, unit, ('b, string) t) Stdlib.format4 -> 'a

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

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

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

val add_ctx : string -> ('a, string) t -> ('a, string) t

add_ctx msg leaves Ok x untouched, but transforms Error s into Error s' where s' contains the additional context given by msg.

  • since 1.2
val add_ctxf : ('a, Stdlib.Format.formatter, unit, ('b, string) t -> ('b, string) t) Stdlib.format4 -> 'a

add_ctxf format_message is similar to add_ctx but with Format for printing the message (eagerly). Example:

add_ctxf "message(number %d, foo: %B)" 42 true (Error "error)"
  • since 1.2
val opt_map : ('a -> ('b, 'c) t) -> 'a option -> ('b option, 'c) t

Map a fallible operation through an option.

  • since 3.7
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

Like 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.

val iter_err : ('err -> unit) -> (_, 'err) t -> unit

Apply the function in case of Error.

  • since 2.4
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.

  • raises Get_error

    if the value is an error.

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

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

val get_or_failwith : ('a, string) t -> 'a

get_or_failwith e returns x if e = Ok x, fails otherwise.

  • raises Failure

    with msg if e = Error msg.

  • since 2.4
val get_lazy : ('b -> 'a) -> ('a, 'b) t -> 'a

get_lazy default_fn x unwraps x, but if x = Error e it returns default_fr e instead.

  • since 3.0
val map_or : ('a -> 'b) -> ('a, 'c) 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 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.

val fold_ok : ('a -> 'b -> 'a) -> 'a -> ('b, _) t -> 'a

fold_ok f acc r will compute f acc x if r=Ok x, and return acc otherwise, as if the result were a mere option.

  • since 1.2
val is_ok : ('a, 'err) t -> bool

Return true if Ok.

  • since 1.0
val is_error : ('a, 'err) t -> bool

Return true if Error.

  • since 1.0

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

Like guard but uses of_exn to print the exception.

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

Like 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

Like guard but gives the function one argument.

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

Like guard but gives the function two arguments.

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

Like guard but gives the function three arguments.

Applicative

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

Synonym of return.

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
include module type of Infix
val (<$>) : ('a -> 'b) -> ('a, 'err) t -> ('b, 'err) t

Infix version of map.

  • since 3.0
val (>|=) : ('a, 'err) t -> ('a -> 'b) -> ('b, 'err) t

Infix version of map with reversed arguments.

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

Monadic composition. e >>= f proceeds as f x if e is Ok x or returns e if e is an Error.

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 let+ : ('a, 'e) t -> ('a -> 'b) -> ('b, 'e) t
  • since 2.8
val and+ : ('a, 'e) t -> ('b, 'e) t -> ('a * 'b, 'e) t
  • since 2.8
val let* : ('a, 'e) t -> ('a -> ('b, 'e) t) -> ('b, 'e) t
  • since 2.8
val and* : ('a, 'e) t -> ('b, 'e) t -> ('a * 'b, 'e) t
  • since 2.8

Collections

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

Same as map_l id: returns Ok [x1;…;xn] if l=[Ok x1; …; Ok xn], or the first error otherwise.

  • since 2.7
val map_l : ('a -> ('b, 'err) t) -> 'a list -> ('b list, 'err) t

map_l f [a1; …; an] applies the function f to a1, …, an ,and, in case of success for every element, returns the list of Ok-value. Otherwise, it fails and returns the first error encountered. Tail-recursive.

val fold_l : ('b -> 'a -> ('b, 'err) t) -> 'b -> 'a list -> ('b, 'err) t
val fold_iter : ('b -> 'a -> ('b, 'err) t) -> 'b -> 'a iter -> ('b, 'err) t
  • since 3.0

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

Convert a result to an option.

val of_opt : 'a option -> ('a, string) t

of_opt opt converts Some v to Ok v and None to Error "of_opt".

val to_iter : ('a, _) t -> 'a iter
  • since 2.8
val to_seq : ('a, _) t -> 'a Stdlib.Seq.t

Renamed from to_std_seq since 3.0.

  • since 3.0
type ('a, 'b) error = [
  1. | `Ok of 'a
  2. | `Error of 'b
]
val of_err : ('a, 'b) error -> ('a, 'b) t
  • since 0.17
val to_err : ('a, 'b) t -> ('a, 'b) error
  • since 0.17

IO

val pp : 'a printer -> ('a, string) t printer
val pp' : 'a printer -> 'e printer -> ('a, 'e) t printer

Printer that is generic on the error type.