1Stdlib.Lazy(3) OCaml library Stdlib.Lazy(3)
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6 Stdlib.Lazy - no description
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9 Module Stdlib.Lazy
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12 Module Lazy
13 : (module Stdlib__lazy)
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21 type 'a t = 'a CamlinternalLazy.t
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24 A value of type 'a Lazy.t is a deferred computation, called a suspen‐
25 sion, that has a result of type 'a . The special expression syntax
26 lazy (expr) makes a suspension of the computation of expr , without
27 computing expr itself yet. "Forcing" the suspension will then compute
28 expr and return its result. Matching a suspension with the special pat‐
29 tern syntax lazy(pattern) also computes the underlying expression and
30 tries to bind it to pattern :
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33 let lazy_option_map f x =
34 match x with
35 | lazy (Some x) -> Some (Lazy.force f x)
36 | _ -> None
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39 Note: If lazy patterns appear in multiple cases in a pattern-matching,
40 lazy expressions may be forced even outside of the case ultimately se‐
41 lected by the pattern matching. In the example above, the suspension x
42 is always computed.
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44 Note: lazy_t is the built-in type constructor used by the compiler for
45 the lazy keyword. You should not use it directly. Always use Lazy.t
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48 Note: Lazy.force is not thread-safe. If you use this module in a
49 multi-threaded program, you will need to add some locks.
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51 Note: if the program is compiled with the -rectypes option, ill-founded
52 recursive definitions of the form let rec x = lazy x or let rec x =
53 lazy(lazy(...(lazy x))) are accepted by the type-checker and lead, when
54 forced, to ill-formed values that trigger infinite loops in the garbage
55 collector and other parts of the run-time system. Without the -rec‐
56 types option, such ill-founded recursive definitions are rejected by
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61 exception Undefined
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67 val force : 'a t -> 'a
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70 force x forces the suspension x and returns its result. If x has al‐
71 ready been forced, Lazy.force x returns the same value again without
72 recomputing it. If it raised an exception, the same exception is
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76 Raises Undefined if the forcing of x tries to force x itself recur‐
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81 val force_val : 'a t -> 'a
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84 force_val x forces the suspension x and returns its result. If x has
85 already been forced, force_val x returns the same value again without
86 recomputing it.
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88 If the computation of x raises an exception, it is unspecified whether
89 force_val x raises the same exception or Lazy.Undefined .
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92 Raises Undefined if the forcing of x tries to force x itself recur‐
93 sively.
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97 val from_fun : (unit -> 'a) -> 'a t
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100 from_fun f is the same as lazy (f ()) but slightly more efficient.
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103 from_fun should only be used if the function f is already defined. In
104 particular it is always less efficient to write from_fun (fun () ->
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108 Since 4.00.0
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112 val from_val : 'a -> 'a t
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115 from_val v returns an already-forced suspension of v . This is for
116 special purposes only and should not be confused with lazy (v) .
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119 Since 4.00.0
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123 val is_val : 'a t -> bool
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126 is_val x returns true if x has already been forced and did not raise an
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130 Since 4.00.0
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134 val lazy_from_fun : (unit -> 'a) -> 'a t
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136 Deprecated. synonym for from_fun .
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140 val lazy_from_val : 'a -> 'a t
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142 Deprecated. synonym for from_val .
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146 val lazy_is_val : 'a t -> bool
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148 Deprecated. synonym for is_val .
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154OCamldoc 2021-07-22 Stdlib.Lazy(3)