Lazy(3)

okay so bestie, welcome to the Lazy(3) page from the OCaml library — we're talking deferred computations, and it is literally giving main character energy. 🦋✨

💖 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. Not me explaining this in the group chat, but okay: 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 — highkey — 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. No cap. 🧚

• val from_fun : (unit -> 'a) -> 'a t — The way that from_fun f slays is: it's 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 — that's the whole module signature, bestie. 🎀

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okay that's the whole page, mwah 💋 — brought to you by OCamldoc (source/version) and dated 2026-06-22. Periodt. 💅✨