Stdlib.Lazy
Deferred computations.
type 'a t = 'a CamlinternalLazy.t
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. Matching a suspension with the special pattern syntax lazy(pattern)
also computes the underlying expression and tries to bind it to pattern
:
let lazy_option_map f x =
match x with
| lazy (Some x) -> Some (Lazy.force f x)
| _ -> None
Note: If lazy patterns appear in multiple cases in a pattern-matching, lazy expressions may be forced even outside of the case ultimately selected by the pattern matching. In the example above, the suspension x
is always computed.
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 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.
map f x
returns a suspension that, when forced, forces x
and applies f
to its value.
It is equivalent to lazy (f (Lazy.force x))
.
val is_val : 'a t -> bool
is_val x
returns true
if x
has already been forced and did not raise an exception.
val from_val : 'a -> 'a t
from_val v
evaluates v
first (as any function would) and returns an already-forced suspension of its result. It is the same as let x = v in lazy x
, but uses dynamic tests to optimize suspension creation in some cases.
map_val f x
applies f
directly if x
is already forced, otherwise it behaves as map f x
.
When x
is already forced, this behavior saves the construction of a suspension, but on the other hand it performs more work eagerly that may not be useful if you never force the function result.
If f
raises an exception, it will be raised immediately when is_val x
, or raised only when forcing the thunk otherwise.
If map_val f x
does not raise an exception, then is_val (map_val f x)
is equal to is_val x
.
The following definitions are for advanced uses only; they require familiary with the lazy compilation scheme to be used appropriately.
val from_fun : (unit -> 'a) -> 'a t
from_fun f
is the same as lazy (f ())
but slightly more efficient.
It 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 force_val : 'a t -> 'a
force_val x
forces the suspension x
and returns its result. If x
has already been forced, force_val x
returns the same value again without recomputing it.
If the computation of x
raises an exception, it is unspecified whether force_val x
raises the same exception or Undefined
.
val lazy_from_fun : (unit -> 'a) -> 'a t
val lazy_from_val : 'a -> 'a t
val lazy_is_val : 'a t -> bool