Base.Lazy
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.
val hash_fold_t : (Hash.state -> 'a -> Hash.state) -> Hash.state -> 'a t -> Hash.state
include Sexpable.S1 with type 'a t := 'a t
val t_of_sexp : (Sexplib0.Sexp.t -> 'a) -> Sexplib0.Sexp.t -> 'a t
val sexp_of_t : ('a -> Sexplib0.Sexp.t) -> 'a t -> Sexplib0.Sexp.t
include Monad.S with type 'a t := 'a t
t >>= f
returns a computation that sequences the computations represented by two monad elements. The resulting computation first does t
to yield a value v
, and then runs the computation returned by f v
.
module Monad_infix : sig ... end
val return : 'a -> 'a t
return v
returns the (trivial) computation that returns v.
ignore_m t
is map t ~f:(fun _ -> ())
. ignore_m
used to be called ignore
, but we decided that was a bad name, because it shadowed the widely used Caml.ignore
. Some monads still do let ignore = ignore_m
for historical reasons.
module Let_syntax : sig ... end
These are convenient to have in scope when programming with a monad:
val force : 'a t -> 'a
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.