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  =  match  x  with  |  lazy (Some x) -> Some
35       (Lazy.force f x) | _ -> None
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37       Note: If lazy patterns appear in multiple cases in a  pattern-matching,
38       lazy  expressions  may  be  forced  even outside of the case ultimately
39       selected by the pattern matching. In the example above, the  suspension
40       x is always computed.
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42       Note:  lazy_t is the built-in type constructor used by the compiler for
43       the lazy keyword.  You should not use it directly.  Always  use  Lazy.t
44       instead.
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46       Note:  Lazy.force  is  not  thread-safe.   If  you use this module in a
47       multi-threaded program, you will need to add some locks.
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49       Note: if the program is compiled with the -rectypes option, ill-founded
50       recursive  definitions  of  the  form let rec x = lazy x or let rec x =
51       lazy(lazy(...(lazy x))) are accepted by the type-checker and lead, when
52       forced, to ill-formed values that trigger infinite loops in the garbage
53       collector and other parts of the run-time system.   Without  the  -rec‐
54       types  option,  such  ill-founded recursive definitions are rejected by
55       the type-checker.
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59       exception Undefined
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65       val force : 'a t -> 'a
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68       force x forces the suspension x and  returns  its  result.   If  x  has
69       already  been forced, Lazy.force x returns the same value again without
70       recomputing it.  If it raised  an  exception,  the  same  exception  is
71       raised  again.  Raise Lazy.Undefined if the forcing of x tries to force
72       x itself recursively.
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76       val force_val : 'a t -> 'a
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79       force_val x forces the suspension x and returns its result.  If  x  has
80       already  been  forced, force_val x returns the same value again without
81       recomputing it.  Raise Lazy.Undefined if the  forcing  of  x  tries  to
82       force  x  itself recursively.  If the computation of x raises an excep‐
83       tion, it is unspecified whether force_val x raises the  same  exception
84       or Lazy.Undefined .
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88       val from_fun : (unit -> 'a) -> 'a t
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91       from_fun f is the same as lazy (f ()) but slightly more efficient.
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94       from_fun  should only be used if the function f is already defined.  In
95       particular it is always less efficient to write  from_fun  (fun  ()  ->
96       expr) than lazy expr .
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99       Since 4.00.0
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103       val from_val : 'a -> 'a t
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106       from_val  v  returns  an  already-forced suspension of v .  This is for
107       special purposes only and should not be confused with lazy (v) .
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110       Since 4.00.0
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114       val is_val : 'a t -> bool
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117       is_val x returns true if x has already been forced and did not raise an
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121       Since 4.00.0
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125       val lazy_from_fun : (unit -> 'a) -> 'a t
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127       Deprecated.  synonym for from_fun .
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131       val lazy_from_val : 'a -> 'a t
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133       Deprecated.  synonym for from_val .
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137       val lazy_is_val : 'a t -> bool
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139       Deprecated.  synonym for is_val .
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145OCamldoc                          2019-07-30                           Lazy(3)