# package ocaml-base-compiler

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.

## Iterators

`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))`

.

## Reasoning on already-forced suspensions

`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`

.

## Advanced

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`

.

## Deprecated

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

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

`val lazy_is_val : 'a t -> bool`