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.
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.
include sig ... end
val hash_fold_t : (Base.Hash.state ‑> 'a ‑> Base.Hash.state) ‑> Base.Hash.state ‑> 'a t ‑> Base.Hash.state
val t_of_sexp : (Base.Sexp.t ‑> 'a) ‑> Base.Sexp.t ‑> 'a t
val sexp_of_t : ('a ‑> Base.Sexp.t) ‑> 'a t ‑> Base.Sexp.t
include Base.Monad.S with type a t := a t
include Base__.Monad_intf.S_without_syntax with type a t := a t
include Base__.Monad_intf.Infix with type a t := a t
module Monad_infix : Base__.Monad_intf.Infix with type a t := a t
external force : 'a t ‑> 'a = "%lazy_force"
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. Raise Undefined
if the
forcing of x
tries to force x
itself recursively.
val force_val : 'a t ‑> 'a
Like force
except that force_val x
does not use an exception handler, so it may be
more efficient. However, if the computation of x
raises an exception, it is
unspecified whether force_val x
raises the same exception or Undefined
.
val from_fun : (unit ‑> 'a) ‑> 'a t
from_fun f
is the same as lazy (f ())
but slightly more efficient if f
is a
variable. from_fun
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
.
val from_val : 'a ‑> 'a t
from_val v
returns an already-forced suspension of v
(where v
can be any
expression). Essentially, from_val expr
is the same as let var = expr in lazy
var
.
val is_val : 'a t ‑> bool
is_val x
returns true
if x
has already been forced and did not raise an
exception.
module T_unforcing : sig ... end
This type offers a serialization function sexp_of_t
that won't force its argument.
Instead, it will serialize the 'a
if it is available, or just use a custom string
indicating it is not forced. Note that this is not a round-trippable type, thus the
type does not expose of_sexp
. To be used in debug code, while tracking a Heisenbug,
etc.