Lazy(3o) OCaml library Lazy(3o) NAME Lazy - Deferred computations. Module Module Lazy Documentation 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 suspen- sion, 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 pat- tern 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 se- lected 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 -rec- types 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 al- ready 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 recur- sively. Iterators 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 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 4.00.0 val from_val : 'a -> 'a t from_val v evaluates v first (as any function would) and returns an al- ready-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 be- haves 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 Advanced The following definitions are for advanced uses only; they require fa- miliary 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 par- ticular 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 recur- sively. Deprecated 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 . OCamldoc 2023-02-12 Lazy(3o)
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