1Lazy(3)                          OCaml library                         Lazy(3)
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NAME

6       Lazy - Deferred computations.
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Module

9       Module   Lazy
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Documentation

12       Module Lazy
13        : sig end
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16       Deferred computations.
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22       type 'a t = 'a lazy_t
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25       A  value  of type 'a Lazy.t is a deferred computation, called a suspen‐
26       sion, that has a result of type 'a .   The  special  expression  syntax
27       lazy  (expr)  makes  a  suspension of the computation of expr , without
28       computing expr itself yet.  "Forcing" the suspension will then  compute
29       expr and return its result.
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31       Note:  lazy_t is the built-in type constructor used by the compiler for
32       the lazy keyword.  You should not use it directly.  Always  use  Lazy.t
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35       Note:  Lazy.force  is  not  thread-safe.   If  you use this module in a
36       multi-threaded program, you will need to add some locks.
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38       Note: if the program is compiled with the -rectypes option, ill-founded
39       recursive  definitions  of  the  form let rec x = lazy x or let rec x =
40       lazy(lazy(...(lazy x))) are accepted by the type-checker and lead, when
41       forced, to ill-formed values that trigger infinite loops in the garbage
42       collector and other parts of the run-time system.   Without  the  -rec‐
43       types  option,  such  ill-founded recursive definitions are rejected by
44       the type-checker.
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48       exception Undefined
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54       val force : 'a t -> 'a
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57       force x forces the suspension x and  returns  its  result.   If  x  has
58       already  been forced, Lazy.force x returns the same value again without
59       recomputing it.  If it raised  an  exception,  the  same  exception  is
60       raised  again.  Raise Lazy.Undefined if the forcing of x tries to force
61       x itself recursively.
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65       val force_val : 'a t -> 'a
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68       force_val x forces the suspension x and returns its result.  If  x  has
69       already  been  forced, force_val x returns the same value again without
70       recomputing it.  Raise Lazy.Undefined if the  forcing  of  x  tries  to
71       force  x  itself recursively.  If the computation of x raises an excep‐
72       tion, it is unspecified whether force_val x raises the  same  exception
73       or Lazy.Undefined .
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77       val from_fun : (unit -> 'a) -> 'a t
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80       from_fun f is the same as lazy (f ()) but slightly more efficient.
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83       from_fun  should only be used if the function f is already defined.  In
84       particular it is always less efficient to write  from_fun  (fun  ()  ->
85       expr) than lazy expr .
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88       Since 4.00.0
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92       val from_val : 'a -> 'a t
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95       from_val  v  returns  an  already-forced suspension of v .  This is for
96       special purposes only and should not be confused with lazy (v) .
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99       Since 4.00.0
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103       val is_val : 'a t -> bool
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106       is_val x returns true if x has already been forced and did not raise an
107       exception.
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110       Since 4.00.0
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114       val lazy_from_fun : (unit -> 'a) -> 'a t
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116       Deprecated.  synonym for from_fun .
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120       val lazy_from_val : 'a -> 'a t
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122       Deprecated.  synonym for from_val .
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126       val lazy_is_val : 'a t -> bool
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128       Deprecated.  synonym for is_val .
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134OCamldoc                          2019-02-02                           Lazy(3)