Lazy − Deferred computations.
Module Lazy
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.
Raises Undefined if the forcing of x tries to force x itself recursively.
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 4.13.0
val is_val : ’a t -> bool
is_val x returns true if x has already been forced and did not raise an exception.
Since 4.00.0
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 4.00.0
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 4.13.0
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 4.00.0
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 .
Raises Undefined if the forcing of x tries to force x itself recursively.
val lazy_from_fun : (unit -> ’a) -> ’a t
Deprecated. synonym for from_fun .
val lazy_from_val : ’a -> ’a t
Deprecated. synonym for from_val .
val lazy_is_val : ’a t -> bool
Deprecated. synonym for is_val .