package containers

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package containers
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Error Monad

The variant is polymorphic in the error type since 0.5

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 = [
  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 fail_printf : ('a, Buffer.t, unit, ('a, string) t) Pervasives.format4 -> 'a

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

  • since 0.3.3
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 error.

  • since 0.5
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

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 Invalid_argument

    if the value is an error.

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 : success:('a -> 'b) -> failure:('err -> 'b) -> ('a, 'err) t -> 'b

fold ~success ~failure e opens e and, if e = `Ok x, returns success x, otherwise e = `Error s and it returns failure s.

Wrappers

The functions guard, wrap1, wrap2 and wrap3 now return exceptions in case of failure,

  • since 0.5
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. See register_printer

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

Applicative

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

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

Global Exception Printers

One can register exception printers here, so they will be used by guard, wrap1, etc. The printers should succeed (print) on exceptions they can deal with, and re-raise the exception otherwise. For instance if I register a printer for Not_found, it could look like:

CCError.register_printer
  (fun buf exn -> match exn with
    | Not_found -> Buffer.add_string buf "Not_found"
    | _ -> raise exn
  );;

This way a printer that doesn't know how to deal with an exception will let other printers do it.

val register_printer : exn printer -> unit