[Libguestfs] [PATCH 1/2] common/mlstdutils: Add ‘return’ statement for OCaml.

This adds a ‘return’ statement as found in other programming languages. You can use it like this: with_return (fun {return} -> some code ... ) where ‘some code’ may either return implicitly (as usual), or may call ‘return x’ to immediately return ‘x’. All returned values must have the same type. The OCaml >= 4.04 implementation is by Petter A. Urkedal and octachron. See this thread: https://sympa.inria.fr/sympa/arc/caml-list/2017-11/msg00017.html The version that works for any OCaml is by me. (Note that my version cannot be nested). --- common/mlstdutils/std_utils.ml | 16 ++++++++++++++++ common/mlstdutils/std_utils.mli | 14 ++++++++++++++ 2 files changed, 30 insertions(+) diff --git a/common/mlstdutils/std_utils.ml b/common/mlstdutils/std_utils.ml index 32bba4113..ba23f39ed 100644 --- a/common/mlstdutils/std_utils.ml +++ b/common/mlstdutils/std_utils.ml @@ -598,6 +598,22 @@ let protect ~f ~finally = finally (); match r with Either ret -> ret | Or exn -> raise exn +type 'a return = { return: 'b. 'a -> 'b } (* OCaml >= 4.03: [@@unboxed] *) +(* This requires features in OCaml >= 4.04: +let with_return (type a) f = + let exception Return of a in + try f {return = fun ret -> raise (Return ret)} with Return ret -> ret +*) + +(* This should work for any version of OCaml, but it doesn't work + * properly for nested with_return statements. When we can assume + * OCaml >= 4.04 we should use the above definition instead. + *) +let with_return f = + let ret = ref None in + try f {return = fun r -> ret := Some r; raise Exit} + with Exit -> match !ret with None -> assert false | Some r -> r + let failwithf fs = ksprintf failwith fs exception Executable_not_found of string (* executable *) diff --git a/common/mlstdutils/std_utils.mli b/common/mlstdutils/std_utils.mli index b3cfdcd55..96c55a511 100644 --- a/common/mlstdutils/std_utils.mli +++ b/common/mlstdutils/std_utils.mli @@ -347,6 +347,20 @@ val protect : f:(unit -> 'a) -> finally:(unit -> unit) -> 'a case, but requires a lot more work by the caller. Perhaps we will change this in future.) *) +type 'a return = { return: 'b. 'a -> 'b } (* OCaml >= 4.03: [@@unboxed] *) +val with_return : ('a return -> 'a) -> 'a +(** {v + with_return (fun {return} -> + some code ... + ) + v} + emulates the [return] statement found in other programming + languages. + + The ‘some code’ part may either return implicitly, or may call + [return x] to immediately return the value [x]. All returned + values must have the same type. *) + val failwithf : ('a, unit, string, 'b) format4 -> 'a (** Like [failwith] but supports printf-like arguments. *) -- 2.13.2