Format.Args(3)

💅 NAME 💅

okay so bestie, this is Format.Args from the OCaml library — and it's literally the module that defines a heterogeneous list type, which you can hand straight to printf-like functions. no cap, that's the whole vibe. 🎀

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🦋 Module 🦋

the module is called Format.Args, and its signature is just : sig end — yes girl, that's the whole thing, it's giving minimalism ✨

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🌸 Documentation 🌸

👑 Module Args

: sig end — iconic, we love a signature that keeps it mysterious 🫶

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so like, the Args module defines a heterogeneous list type, and you use it as the argument of printf-like functions. that's it, that's the function. 💖

here's the part that's lowkey the best: you do not need to open this module when you're using the functions that accept an Args.t. thanks to type-based disambiguation, the type gets inferred automatically — so the list syntax [x; y; z] works directly for heterogeneous lists. no open, no ceremony, just vibes. 🧚✨

seriously, don't open it, it's not required — the compiler just gets you. iykyk 💅

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🌈 an example 🌈

(* without opening Args *)
Format.lprintf "%s %d %.02f@." [ "ocaml"; 42; 3.14 ]

(* or with explicit construction *)
let lst = let open Format.Args in "ocaml" :: [ 42; 3.14 ] @ [ 'c' ] in
Format.lprintf "%s %d %.02f %c@." lst

see how the first one just... works? three totally different types in one list, no drama. and the second one shows you the explicit-construction route if you're feeling main character energy. both ate. 🔥

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🍓 Since 5.5 🍓

this dropped in 5.5, so if you're on something older, sorry bestie, no list magic for you 😭

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⭐ the type, fr fr ⭐

type ('a, 'r) t =
 | [] : ('r0, 'r0) t
 | (::) : 'a0 * ('b, 'r1) t -> ('a0 -> 'b, 'r1) t

okay look at that GADT. [] is the empty case, and (::) conses one element 'a0 onto a tail and the whole thing becomes ('a0 -> 'b, 'r1) t. it's literally building up a function type, one element at a time. the way that this type-checks... obsessed. 💜

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🎀 the values 🎀

• val apply : 'a -> ('a, 'r) t -> 'r — you give it a value and an Args.t, and it gives you back an 'r. that's the apply, bestie, that's the whole plot 🦋
• val (@) : ('a, 'r1) t -> ('r1, 'r2) t -> ('a, 'r2) t — the append operator, concatenating two Args.t-shaped things into one, threading 'r1 through the middle. slay. ✨

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💀 reminder: @ here is the Format.Args one, working on Args.t values — don't go confusing it with the list append you already know, they're different girlies.

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and that's the whole page! love that for us. periodt. 🌈

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this lil sign-off is brought to you by OCamldoc, dated 2026-06-22, serving you Format.Args(3) realness 🎀✨💖