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.

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.

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. Raise Undefined if the forcing of x tries to force x itself recursively. If the computation of x raises an exception, it is unspecified whether force_val x raises the same exception or Undefined.

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.

• since 4.00.0

from_val v returns an already-forced suspension of v. This is for special purposes only and should not be confused with lazy (v).

• since 4.00.0

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

• since 4.00.0

• deprecated

  synonym for from_fun.

• deprecated

  synonym for from_val.

• deprecated

  synonym for is_val.