---
name: "Lazy"
section: "3o"
platform: "linux"
package: "ocaml"
version: "5.5.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/Lazy"
---

# Lazy(3o)

Deferred computations.

## NAME

Lazy - Deferred computations.

## Module

Module   Lazy

## Documentation

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 concurrency-safe. If you use this module with multiple fibers, systhreads or domains, then you will need to add some locks. The module however ensures memory-safety, and hence, concurrently accessing this module will not lead to a crash but the behaviour is unspecified.

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*

Raised when forcing a suspension concurrently from multiple fibers, systhreads or domains, or when the suspension tries to force itself recursively.

*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** (see  **Lazy.Undefined** ).

### Iterators

*val map* :  **('a -\> 'b) -\> 'a t -\> 'b t**

**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))** .

**Since** 4.13

### 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.

**Since** 4.00

*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.

**Since** 4.00

*val map\_val* :  **('a -\> 'b) -\> 'a t -\> 'b t**

**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** .

**Since** 4.13

### Advanced

The following definitions are for advanced uses only; they require familiarity 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** .

**Since** 4.00

*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.

**Raises Undefined** (see  **Lazy.Undefined** ).

*module Mutexed :* **sig end**