The Toplevel (REPL) and Your First Program

3.1 Using utop

;; ends a toplevel phrase (only needed in the REPL, almost never in files).

Directives start with #: #use "file.ml";;, #require "str";;, #show List;;, #typeof "x";;, #trace f;;, #quit;;.

$ utop
# 1 + 2;;
- : int = 3
# let greeting = "Hello";;
val greeting : string = "Hello"
# #quit;;

3.2 Hello World

Compile & run:

let () = ... pattern-matches the result against unit (), which acts as the program's "main" and ensures the expression has type unit.

let () = print_endline "Hello, World!"

ocaml hello.ml                 # interpret directly
ocamlc -o hello hello.ml       # bytecode executable
ocamlopt -o hello hello.ml     # native executable
./hello

3.3 Hello World with Dune

dune init project hello
cd hello
dune build
dune exec hello

hello/
├── bin/
│   ├── dune          (executable (public_name hello) (name main) (libraries hello))
│   └── main.ml
├── lib/
│   └── dune          (library (name hello))
├── test/
│   ├── dune
│   └── test_hello.ml
├── dune-project
└── hello.opam

3.4 Comments

There are no single-line // comments.

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