Getting Started

Rex programs are 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

From this repository, you can run a file:

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

Or evaluate a small snippet inline:

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

What you should see

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

What “one expression” means

Even with declarations, the program result is the final expression:

fn inc : i32 -> i32 = \x -> 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 {- ... -}:

{- This is a comment -}
1 + 2

Whitespace and layout

Most whitespace is insignificant, but some constructs are easiest to read in “layout style”:

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

Commas between let bindings are required. The parser also accepts many one-line forms, but multi-line layout tends to be easier to debug.

Type-class and instance method blocks are also written by indentation:

class Size a
  size : a -> i32

You may also see the optional where keyword in class/instance headers:

class Size a where
  size : a -> i32

Both forms are accepted.

Your first “real” Rex file

Create a file hello.rex:

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

Run it:

cargo run -p rex -- run hello.rex

Unicode (λ, →) versus ASCII

Rex accepts both:

• \ and ->
• λ and →

Use whichever your editor makes pleasant. The repo’s examples use a mix.