Module Base.Lazy

A value of type 'a Lazy.t is a deferred computation, called a suspension, that has a result of type 'a.

The special expression syntax lazy (expr) makes a suspension of the computation of expr, without computing expr itself yet. "Forcing" the suspension will then compute expr and return its result.

Note: lazy_t is the built-in type constructor used by the compiler for the lazy keyword. You should not use it directly. Always use Lazy.t instead.

Note: Lazy.force is not thread-safe. If you use this module in a multi-threaded program, you will need to add some locks.

Note: if the program is compiled with the -rectypes option, ill-founded recursive definitions of the form let rec x = lazy x or let rec x = lazy(lazy(...(lazy x))) are accepted by the type-checker and lead, when forced, to ill-formed values that trigger infinite loops in the garbage collector and other parts of the run-time system. Without the -rectypes option, such ill-founded recursive definitions are rejected by the type-checker.

type 'a t = 'a lazy_t
include sig ... end
val compare : ('a ‑> 'a ‑> int) ‑> 'a t ‑> 'a t ‑> int
val hash_fold_t : (Hash.state ‑> 'a ‑> Hash.state) ‑> Hash.state ‑> 'a t ‑> Hash.state
val t_of_sexp : (Sexp.t ‑> 'a) ‑> Sexp.t ‑> 'a t
val sexp_of_t : ('a ‑> Sexp.t) ‑> 'a t ‑> Sexp.t
include Monad.S with type a t := a t
type 'a t
include Base__.Monad_intf.S_without_syntax with type a t := a t
type 'a t
include Base__.Monad_intf.Infix with type a t := a t
type 'a t
val (>>=) : 'a t ‑> ('a ‑> 'b t) ‑> 'b t

t >>= f returns a computation that sequences the computations represented by two monad elements. The resulting computation first does t to yield a value v, and then runs the computation returned by f v.

val (>>|) : 'a t ‑> ('a ‑> 'b) ‑> 'b t

t >>| f is t >>= (fun a -> return (f a)).

module Monad_infix : Base__.Monad_intf.Infix with type a t := a t
val bind : 'a t ‑> f:('a ‑> 'b t) ‑> 'b t

bind t ~f = t >>= f

val return : 'a ‑> 'a t

return v returns the (trivial) computation that returns v.

val map : 'a t ‑> f:('a ‑> 'b) ‑> 'b t

map t ~f is t >>| f.

val join : 'a t t ‑> 'a t

join t is t >>= (fun t' -> t').

val ignore_m : 'a t ‑> unit t

ignore_m t is map t ~f:(fun _ -> ()). ignore_m used to be called ignore, but we decided that was a bad name, because it shadowed the widely used Pervasives.ignore. Some monads still do let ignore = ignore_m for historical reasons.

val all : 'a t list ‑> 'a list t
val all_unit : unit t list ‑> unit t
val all_ignore : unit t list ‑> unit t
  • Deprecated [since 2018-02] Use [all_unit]
include Base__.Monad_intf.Syntax with type a t := a t
type 'a t
module Let_syntax : sig ... end
exception Undefined
external force : 'a t ‑> 'a = "%lazy_force"

force x forces the suspension x and returns its result. If x has already been forced, Lazy.force x returns the same value again without recomputing it. If it raised an exception, the same exception is raised again. Raise Undefined if the forcing of x tries to force x itself recursively.

val force_val : 'a t ‑> 'a

Like force except that force_val x does not use an exception handler, so it may be more efficient. However, if the computation of x raises an exception, it is unspecified whether force_val x raises the same exception or Undefined.

val from_fun : (unit ‑> 'a) ‑> 'a t

from_fun f is the same as lazy (f ()) but slightly more efficient if f is a variable. from_fun should only be used if the function f is already defined. In particular it is always less efficient to write from_fun (fun () -> expr) than lazy expr.

val from_val : 'a ‑> 'a t

from_val v returns an already-forced suspension of v (where v can be any expression). Essentially, from_val expr is the same as let var = expr in lazy var.

val is_val : 'a t ‑> bool

is_val x returns true if x has already been forced and did not raise an exception.

module T_unforcing : sig ... end

This type offers a serialization function sexp_of_t that won't force its argument. Instead, it will serialize the 'a if it is available, or just use a custom string indicating it is not forced. Note that this is not a round-trippable type, thus the type does not expose of_sexp. To be used in debug code, while tracking a Heisenbug, etc.