package ocaml-base-compiler

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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 concurrency-safe. If you use this module with multiple fibers, systhreads or domains, then you will need to add some locks. The module however ensures memory-safety, and hence, concurrently accessing this module will not lead to a crash but the behaviour is unspecified.

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

Raised when forcing a suspension concurrently from multiple fibers, systhreads or domains, or when the suspension tries to force itself recursively.

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.


val map : ('a -> 'b) -> 'a t -> 'b t

map f x returns a suspension that, when forced, forces x and applies f to its value.

It is equivalent to lazy (f (Lazy.force x)).

  • since 4.13

Reasoning on already-forced suspensions

val is_val : 'a t -> bool

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

  • since 4.00
val from_val : 'a -> 'a t

from_val v evaluates v first (as any function would) and returns an already-forced suspension of its result. It is the same as let x = v in lazy x, but uses dynamic tests to optimize suspension creation in some cases.

  • since 4.00
val map_val : ('a -> 'b) -> 'a t -> 'b t

map_val f x applies f directly if x is already forced, otherwise it behaves as map f x.

When x is already forced, this behavior saves the construction of a suspension, but on the other hand it performs more work eagerly that may not be useful if you never force the function result.

If f raises an exception, it will be raised immediately when is_val x, or raised only when forcing the thunk otherwise.

If map_val f x does not raise an exception, then is_val (map_val f x) is equal to is_val x.

  • since 4.13


The following definitions are for advanced uses only; they require familiary with the lazy compilation scheme to be used appropriately.

val from_fun : (unit -> 'a) -> 'a t

from_fun f is the same as lazy (f ()) but slightly more efficient.

It 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
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 Undefined.

  • raises Undefined

    if the forcing of x tries to force x itself recursively.


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