package lwt

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Explicit error handling

  • since 2.6.0

This module provides helpers for values of type ('a, 'b) result Lwt.t. The module is experimental and may change in the future.

type (+'a, +'b) t = ('a, 'b) Result.result Lwt.t
val return : 'a -> ('a, _) t
val fail : 'b -> (_, 'b) t
val lift : ('a, 'b) Result.result -> ('a, 'b) t
val ok : 'a Lwt.t -> ('a, _) t
val catch : 'a Lwt.t -> ('a, exn) t

catch x behaves like return y if x evaluates to y, and like fail e if x raises e

val get_exn : ('a, exn) t -> 'a Lwt.t

get_exn is the opposite of catch: it unwraps the result type, returning the value in case of success, calls in case of error.

val map : ('a -> 'b) -> ('a, 'e) t -> ('b, 'e) t
val map_err : ('e1 -> 'e2) -> ('a, 'e1) t -> ('a, 'e2) t
val bind : ('a, 'e) t -> ('a -> ('b, 'e) t) -> ('b, 'e) t
val bind_lwt : ('a, 'e) t -> ('a -> 'b Lwt.t) -> ('b, 'e) t
val bind_lwt_err : ('a, 'e1) t -> ('e1 -> 'e2 Lwt.t) -> ('a, 'e2) t
val bind_result : ('a, 'e) t -> ('a -> ('b, 'e) Result.result) -> ('b, 'e) t
val both : ('a, 'e) t -> ('b, 'e) t -> ('a * 'b, 'e) t

Lwt.both p_1 p_2 returns a promise that is pending until both promises p_1 and p_2 become resolved. If only p_1 is Error e, the promise is resolved with Error e, If only p_2 is Error e, the promise is resolved with Error e, If both p_1 and p_2 resolve with Error _, the promise is resolved with the error that occurred first.

module Infix : sig ... end
module Let_syntax : sig ... end
module Syntax : sig ... end
include module type of Infix
val (>|=) : ('a, 'e) t -> ('a -> 'b) -> ('b, 'e) t
val (>>=) : ('a, 'e) t -> ('a -> ('b, 'e) t) -> ('b, 'e) t

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