Obj(3)

✨ NAME ✨

okay so bestie, this is Obj from the OCaml library manual, section 3 — Operations on internal representations of values. It's lowkey giving "expert internals only" energy, and the page literally says Not for the casual user. 💅

🧚 Module 🧚

Module Obj

💖 Documentation 💖

Module Obj : sig end

Operations on internal representations of values.

Not for the casual user. No cap, this is advanced internals territory. 🫶

🌸 Types 🌸

• type t
• type raw_data = nativeint

💖 Values and functions 💖

here's the signature stack, exactly as written — the page doesn't add extra descriptions for most of these, so I'm not inventing lore:

• val repr : 'a -> t
• val obj : t -> 'a
• val magic : 'a -> 'b
• val is_block : t -> bool
• val is_int : t -> bool
• val tag : t -> int
• val size : t -> int
• val reachable_words : t -> int — Computes the total size (in words, including the headers) of all heap blocks accessible from the argument. Statically allocated blocks are included. Since 4.04 ✨
• val field : t -> int -> t
• val set_field : t -> int -> t -> unit

When using flambda: set_field MUST NOT be called on immutable blocks. (Blocks allocated in C stubs, or with new_block below, are always considered mutable.) The same goes for set_double_field. For experts only: set_field et al can be made safe by first wrapping the block in Sys.opaque_identity, so any information about its contents will not be propagated.

• val double_field : t -> int -> float
• val set_double_field : t -> int -> float -> unit
• val raw_field : t -> int -> raw_data
• val set_raw_field : t -> int -> raw_data -> unit
• val new_block : int -> int -> t
• val dup : t -> t
• val add_offset : t -> Int32.t -> t
• val with_tag : int -> t -> t

🎀 Tag constants 🎀

all int constants, in exact page order:

• val first_non_constant_constructor_tag : int
• val last_non_constant_constructor_tag : int
• val forcing_tag : int
• val cont_tag : int
• val lazy_tag : int
• val closure_tag : int
• val object_tag : int
• val infix_tag : int
• val forward_tag : int
• val no_scan_tag : int
• val abstract_tag : int
• val string_tag : int
• val double_tag : int
• val double_array_tag : int
• val custom_tag : int
• val int_tag : int
• val out_of_heap_tag : int
• val unaligned_tag : int

🦋 Submodules 🦋

• module Extension_constructor : sig end
• module Ephemeron : sig end

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okay that's the whole Obj(3) page, bestie. This rewrite is sourced from OCamldoc, dated 2026-06-22, for Obj(3) — go be iconic with your internals, but maybe don't @ me if you crash the runtime 💖✨