module Lazy: sig .. end
Deferred computations.
type 'a t = 'a CamlinternalLazy.t
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. Matching a suspension with the special pattern syntax lazy(pattern)
also computes the underlying expression and tries to bind it to pattern
:
let lazy_option_map f x = match x with | lazy (Some x) -> Some (Lazy.force f x) | _ -> None
Note: If lazy patterns appear in multiple cases in a pattern-matching, lazy expressions may be forced even outside of the case ultimately selected by the pattern matching. In the example above, the suspension x
is always computed.
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.
exception Undefined
val force : 'a t -> 'a
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.
Undefined
if the forcing of x
tries to force x
itself recursively.val force_val : 'a t -> 'a
force_val x
forces the suspension x
and returns its result. If x
has already been forced, force_val x
returns the same value again without recomputing it.
If the computation of x
raises an exception, it is unspecified whether force_val x
raises the same exception or Lazy.Undefined
.
Undefined
if the forcing of x
tries to force x
itself recursively.val from_fun : (unit -> 'a) -> 'a t
from_fun f
is the same as lazy (f ())
but slightly more efficient.
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
. This is for special purposes only and should not be confused with lazy (v)
.
val is_val : 'a t -> bool
is_val x
returns true
if x
has already been forced and did not raise an exception.
val lazy_from_fun : (unit -> 'a) -> 'a t
from_fun
.val lazy_from_val : 'a -> 'a t
from_val
.val lazy_is_val : 'a t -> bool
is_val
.
© INRIA 1995-2020.
https://www.ocaml.org/releases/4.11/htmlman/libref/Lazy.html