Getting Started

Small Rex programs can be written as one expression, optionally preceded by top-level declarations:

• type — algebraic data types (ADTs)
• class / instance — type classes and instances
• fn — top-level functions

Note: This tutorial focuses on writing Rex code. If you want to embed Rex in Rust, see Embedding.

Running Rex

A runnable Rex file either defines main or ends with a final expression. The examples in this tutorial usually use final expressions because they keep small programs compact.

From this repository, you can run a Rex file:

cargo run -p rex-cli --bin rex -- rex-cli/examples/record_update.rex

Or evaluate a small snippet inline:

cargo run -p rex-cli --bin rex -- -c 'map ((*) 2) [1, 2, 3]'

What you should see

The CLI prints the evaluated value of the program entry point in JSON format. If something fails, you’ll get a parse/type/eval error (often with a span).

What “one expression” means

Even with declarations, a program without main uses the final expression as its result:

fn inc x: i32 -> i32 = x + 1;

let xs = [1, 2, 3] in
  map inc xs

The program above:

1. Declares a top-level function inc.
2. Creates a list xs.
3. Evaluates map inc xs as the program’s result.

Comments

Comments use // ... for line comments and /* ... */ for block comments:

/* This is a comment */
1 + 2

Whitespace

Most whitespace is insignificant, and indentation has no syntactic meaning. Multi-line expressions are often easier to read:

let
  x = 1,
  y = 2
in
  x + y

Commas between let bindings are required. Top-level function declarations end with semicolons, so multi-line bodies do not depend on indentation. The parser also accepts many one-line forms, but multi-line formatting tends to be easier to debug.

Type-class and instance method blocks use explicit braces and semicolon-separated methods:

class Size a where {
  size : a -> i32;
}

An empty marker class or instance uses a semicolon:

class Marker a;
instance Marker i32;

Your first “real” Rex file

Create a file hello.rex:

let
  greet = \name -> "hello, " + name
in
  greet "world"

Run it:

cargo run -p rex-cli --bin rex -- hello.rex

Lambda and Arrow Spelling

Rex uses ASCII-only syntax for lambdas and arrows:

• \ and ->

The Unicode lambda and right-arrow glyphs are not accepted.