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 CamlinternalLazy.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. Matching a suspension with the special pat‐
30       tern syntax lazy(pattern) also computes the underlying  expression  and
31       tries to bind it to pattern :
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34           let lazy_option_map f x =
35           match x with
36           | lazy (Some x) -> Some (Lazy.force f x)
37           | _ -> None
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40       Note:  If lazy patterns appear in multiple cases in a pattern-matching,
41       lazy expressions may be forced even outside of the case ultimately  se‐
42       lected  by the pattern matching. In the example above, the suspension x
43       is always computed.
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45       Note: lazy_t is the built-in type constructor used by the compiler  for
46       the  lazy  keyword.  You should not use it directly.  Always use Lazy.t
47       instead.
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49       Note: Lazy.force is not thread-safe.  If  you  use  this  module  in  a
50       multi-threaded program, you will need to add some locks.
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52       Note: if the program is compiled with the -rectypes option, ill-founded
53       recursive definitions of the form let rec x = lazy x or  let  rec  x  =
54       lazy(lazy(...(lazy x))) are accepted by the type-checker and lead, when
55       forced, to ill-formed values that trigger infinite loops in the garbage
56       collector  and  other  parts of the run-time system.  Without the -rec‐
57       types option, such ill-founded recursive definitions  are  rejected  by
58       the type-checker.
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62       exception Undefined
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68       val force : 'a t -> 'a
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71       force  x  forces the suspension x and returns its result.  If x has al‐
72       ready been forced, Lazy.force x returns the same  value  again  without
73       recomputing  it.   If  it  raised  an  exception, the same exception is
74       raised again.
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77       Raises Undefined if the forcing of x tries to  force  x  itself  recur‐
78       sively.
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82       val force_val : 'a t -> 'a
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85       force_val  x  forces the suspension x and returns its result.  If x has
86       already been forced, force_val x returns the same value  again  without
87       recomputing it.
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89       If  the computation of x raises an exception, it is unspecified whether
90       force_val x raises the same exception or Lazy.Undefined .
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93       Raises Undefined if the forcing of x tries to  force  x  itself  recur‐
94       sively.
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98       val from_fun : (unit -> 'a) -> 'a t
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101       from_fun f is the same as lazy (f ()) but slightly more efficient.
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104       from_fun  should only be used if the function f is already defined.  In
105       particular it is always less efficient to write  from_fun  (fun  ()  ->
106       expr) than lazy expr .
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109       Since 4.00.0
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113       val from_val : 'a -> 'a t
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116       from_val  v  returns  an  already-forced suspension of v .  This is for
117       special purposes only and should not be confused with lazy (v) .
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120       Since 4.00.0
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124       val is_val : 'a t -> bool
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127       is_val x returns true if x has already been forced and did not raise an
128       exception.
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131       Since 4.00.0
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135       val lazy_from_fun : (unit -> 'a) -> 'a t
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137       Deprecated.  synonym for from_fun .
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141       val lazy_from_val : 'a -> 'a t
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143       Deprecated.  synonym for from_val .
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147       val lazy_is_val : 'a t -> bool
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149       Deprecated.  synonym for is_val .
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155OCamldoc                          2021-01-26                           Lazy(3)