Ephemeron.S(3)

💖 Ephemeron.S(3) — the OCaml library glow-up ✨

ok so picture this: you're deep in the OCaml library manual, page Ephemeron.S(3), and you land on this baddie. Welcome bestie, let me walk you through it 🫶

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🌸 NAME

Ephemeron.S — literally the output signature of the functors Ephemeron.K1.Make and Ephemeron.K2.Make. that's it, that's the vibe 💅

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🎀 Module type

Module type Ephemeron.S

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📖 Documentation

🦋 Module type S

module type S
 = sig end

So what is this? It's the output signature of the functors Ephemeron.K1.Make and Ephemeron.K2.Make ✨ These hash tables are weak in the keys — meaning: if all the keys of a binding are alive, the binding is kept. But the second one of the keys of that binding is dead? the binding gets removed. gone. deleted. no cap 💀

It proposes the same interface as your usual hash table, which sounds slay, BUT — and this is the important part, like, listen to me — since the bindings are weak, even if mem h k is true, a subsequent find h k may raise Not_found, because the garbage collector can run between the two. 😭 the GC is the main character here and it does not care about your plans.

🚨 reminder bestie: mem h k being true does not promise find h k will succeed. the GC may have slayed that binding in between. not a bug, just the ephemeron lifestyle.

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💜 The types

• type key — the key type. iconic, mysterious, doesn't elaborate 👑
• type !'a t — the table type itself, with that ! variance annotation sitting there looking all fancy ⭐

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🔥 The values, one by one

• val create : int -> 'a t — make a new table, you know the drill 💖
• val clear : 'a t -> unit — wipe the whole table 🧹
• val reset : 'a t -> unit — reset it, another clean slate energy ✨
• val copy : 'a t -> 'a t — duplicate the table, twin moment 🦋
• val add : 'a t -> key -> 'a -> unit — add a binding, put it in 🎀
• val remove : 'a t -> key -> unit — take that binding out 🙈
• val find : 'a t -> key -> 'a — grab the value for a key (or Not_found, see the warning above 😭)
• val find_opt : 'a t -> key -> 'a option — same but wrapped in an option, the gentler version 🫶
• val find_all : 'a t -> key -> 'a list — all the values for that key, in a list ✨
• val replace : 'a t -> key -> 'a -> unit — swap the binding, out with the old, in with the new 💅
• val mem : 'a t -> key -> bool — is this key in there? (true doesn't guarantee a later find, remember 💀)
• val length : 'a t -> int — how many bindings, a number, iconic 🌈
• val stats : 'a t -> Hashtbl.statistics — hashtable stats, the receipts 📊
• val add_seq : 'a t -> (key * 'a) Seq.t -> unit — add a whole sequence of bindings, bulk slay 🔥
• val replace_seq : 'a t -> (key * 'a) Seq.t -> unit — replace from a sequence, same energy 👑
• val of_seq : (key * 'a) Seq.t -> 'a t — build a whole fresh table from a sequence, chef's kiss 🍓
• val clean : 'a t -> unit — remove all dead bindings. Done automatically during automatic resizing, so honestly it does this itself, but you can call it 💅
• val stats_alive : 'a t -> Hashtbl.statistics — same as Hashtbl.SeededS.stats but only count the alive bindings ⭐ like a stats call that did a vibe check first

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🫶 quick recap: weak keys, same interface as a normal hash table, GC can delete your bindings mid-thought, and clean / stats_alive are your friends for dealing with the fallout.

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that's the whole page bestie, every type, every val, every warning — nothing skipped, we ate and left no crumbs 🎀

sign-off 💌: this lil page came from OCamldoc, dated 2026-06-22, on the Ephemeron.S(3) page in section 3 of the manual. mwah, go write some ephemerons 💜