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 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.

   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 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

   Advanced
       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.

   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 .