Learn

OCaml's Printf is type-checked at compile time. The Format module goes further — pretty-printing with boxes, indentation, and line breaks.

open Format

let () =
    printf "@[<v 2>Result:@,%d@,%s@]@." 42 "hello"

Format directives in @:

• @[<v 2> — open vertical box, indent 2
• @[<h> — horizontal box (no breaks)
• @[<hv 2> — horizontal-or-vertical (try horizontal, break to vertical if too wide)
• @, — break point (line break in vertical, space in horizontal)
• @ — break or space
• @] — close box
• @. — newline + flush

Pretty printing custom types:

type tree = Leaf | Node of int * tree * tree

let rec pp_tree fmt = function
    | Leaf -> Format.pp_print_string fmt "leaf"
    | Node (v, l, r) ->
        Format.fprintf fmt "@[<v 2>node %d@,%a@,%a@]" v pp_tree l pp_tree r

let () =
    let t = Node (1, Node (2, Leaf, Leaf), Node (3, Leaf, Leaf)) in
    Format.printf "%a@." pp_tree t

Outputs:

node 1
  node 2
    leaf
    leaf
  node 3
    leaf
    leaf

%a is the directive for "call this printer."

Buffer module for string building:

let buf = Buffer.create 256
Buffer.add_string buf "Hello, ";
Buffer.add_string buf "World!";
Buffer.add_char buf '\n';
print_string (Buffer.contents buf)

Far more efficient than repeated ^ concatenation for large strings — ^ is O(n+m) every call.

Printf.sprintf for one-shot formatting:

let greeting = Printf.sprintf "Hello, %s, age %d" name age

Type-safe format strings — %d requires int, etc. If types don't match, compile error:

Printf.printf "%d" "hello"   (* compile error — %d wants int *)

Most OCaml standard library reaches for Format (e.g., Format.printf from Pervasives). Real packages provide custom pp_* functions for their types.