package out-channel-redirect

out-channel-redirect

An OCaml library for redirecting and capturing output written to out_channels, with support for native, JavaScript (js_of_ocaml), and WebAssembly (wasm_of_ocaml) targets.

On native, it works by redirecting file descriptors at the OS level (via dup/dup2), so it captures output from OCaml code as well as C stubs and foreign calls. On JavaScript and WebAssembly targets, it redirects at the channel level using the runtime's channel primitives.

Installation

opam install out-channel-redirect

Usage

(* Capture an out_channel into a string *)
let output, result = Out_channel_redirect.capture_channel stdout ~f:(fun () ->
    print_endline "hello";
    42)
(* output = "hello\n", result = 42 *)

(* Convenience shorthands for stdout, stderr, or both *)
let output, result = Out_channel_redirect.capture_stdout ~f:(fun () -> print_endline "hello"; 42)
let output, result = Out_channel_redirect.capture_stderr ~f:(fun () -> prerr_endline "hello"; 42)
let output, result = Out_channel_redirect.capture_outputs_interleaved ~f:(fun () ->
    print_endline "hello"; prerr_endline "world"; 42)

Redirecting a channel into another

redirect redirects a channel into another without capturing to a string:

Out_channel_redirect.redirect stderr ~into:stdout ~f:(fun () ->
    prerr_endline "this goes to stdout now")

Expert API

For manual control over redirection lifetime:

let redir = Out_channel_redirect.Expert.redirect ~into:stdout stderr in
prerr_endline "redirected to stdout";
Out_channel_redirect.Expert.stop redir

API

val redirect : out_channel -> into:out_channel -> f:(unit -> 'a) -> 'a

val capture_channel : out_channel -> f:(unit -> 'a) -> string * 'a
val capture_channels : out_channel list -> f:(unit -> 'a) -> string list * 'a
val capture_channels_interleaved :
  out_channel list -> f:(unit -> 'a) -> string * 'a

val capture_stdout : f:(unit -> 'a) -> string * 'a
val capture_stderr : f:(unit -> 'a) -> string * 'a
val capture_outputs : f:(unit -> 'a) -> string * string * 'a
val capture_outputs_interleaved : f:(unit -> 'a) -> string * 'a
val capture : f:(unit -> 'a) -> string * 'a
(* alias for capture_outputs_interleaved *)

module Expert : sig
  type redirection
  val redirect : into:out_channel -> out_channel -> redirection
  val stop : redirection -> unit
end

Requirements

• OCaml >= 4.11
• Dune >= 3.17

License

MIT