Module Base.With_return

with_return f allows for something like the return statement in C within f.

There are three ways f can terminate:

  1. If f calls r.return x, then x is returned by with_return.
  2. If f evaluates to a value x, then x is returned by with_return.
  3. If f raises an exception, it escapes with_return.

Here is a typical example:

let find l ~f =
  with_return (fun r ->
    List.iter l ~f:(fun x -> if f x then r.return (Some x));
    None
  )

It is only because of a deficiency of ML types that with_return doesn't have type:

val with_return : 'a. (('a -> ('b. 'b)) -> 'a) -> 'a

but we can slightly increase the scope of 'b without changing the meaning of the type, and then we get:

type 'a return = { return : 'b . 'a -> 'b }
val with_return : ('a return -> 'a) -> 'a

But the actual reason we chose to use a record type with polymorphic field is that otherwise we would have to clobber the namespace of functions with return and that is undesirable because return would get hidden as soon as we open any monad. We considered names different than return but everything seemed worse than just having return as a record field. We are clobbering the namespace of record fields but that is much more acceptable.

type -'a return = private {
return : b. 'a -> 'b;
}
val with_return : ('a return -> 'a) -> 'a
val with_return_option : ('a return -> unit) -> 'a option

Note that with_return_option allocates ~5 words more than the equivalent with_return call.

val prepend : 'a return -> f:('b -> 'a) -> 'b return

prepend a ~f returns a value x such that each call to x.return first applies f before applying a.return. The call to f is "prepended" to the call to the original a.return. A possible use case is to hand x over to another function which returns 'b, a subtype of 'a, or to capture a common transformation f applied to returned values at several call sites.