module Lazy:`sig`

..`end`

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.

`exception Undefined`

`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.

**Raises**`Undefined`

if the forcing of`x`

tries to force`x`

itself recursively.

`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 `Lazy.Undefined`

.

**Raises**`Undefined`

if the forcing of`x`

tries to force`x`

itself recursively.

`val from_fun : ``(unit -> 'a) -> 'a t`

`from_fun f`

is the same as `lazy (f ())`

but slightly more efficient.

`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`

.

**Since**4.00.0

`val from_val : ``'a -> 'a t`

`from_val v`

returns an already-forced suspension of `v`

.
This is for special purposes only and should not be confused with
`lazy (v)`

.

**Since**4.00.0

`val is_val : ``'a t -> bool`

`is_val x`

returns `true`

if `x`

has already been forced and
did not raise an exception.

**Since**4.00.0

`val lazy_from_fun : ``(unit -> 'a) -> 'a t`

Deprecated.synonym for

`from_fun`

.`val lazy_from_val : ``'a -> 'a t`

Deprecated.synonym for

`from_val`

.`val lazy_is_val : ``'a t -> bool`

Deprecated.synonym for

`is_val`

.