A Tour of the Prelude

Rex ships with a small “prelude” of standard types, type classes, and instances.

The source of truth in this repository is:

• type classes + instances: rexlang-typesystem/src/prelude_typeclasses.rex
• built-in types + helper functions: rexlang-typesystem/src/prelude.rs

This page is a guided map so you know what to reach for while writing Rex.

Core data types

These are available by default:

• List a (with constructors Empty and Cons)
• Option a (constructors Some and None)
• Result a e (constructors Err and Ok)
• Array a
• Dict a

Core classes (selected)

Numeric-like classes

• AdditiveMonoid a (zero, +)
• MultiplicativeMonoid a (one, *)
• Ring a, Field a, Integral a (and friends)

Equality and ordering

• Eq a (==, !=)
• Ord a (cmp, <, <=, >, >=)
• Default a (default)

Default gives you a value-level default for a type. It is separate from Rex’s defaulting pass, which resolves ambiguous type variables for defaultable classes.

Collections and effects

• Functor f (map)
• Applicative f (pure, ap)
• Monad m (bind)
• Foldable t (foldl, foldr, fold)
• Filterable f (filter, filter_map)
• Sequence f (take, skip, zip, unzip)
• Indexable t a (get)

For tuples, use numeric projection like .0 and .1 instead of get.

A few useful helper functions

The prelude also exposes some generic helpers (type-class-based):

• sum, mean, count, min, max
• is_some, is_none (for Option)
• is_ok, is_err (for Result)

How to learn what something is

When you see an unfamiliar function:

1. Ask the CLI for its type: cargo run -p rex -- run --emit-type -c 'the_name'
2. If it’s a type-class method, find the class in rexlang-typesystem/src/prelude_typeclasses.rex
3. If it’s a helper function, find it in rexlang-typesystem/src/prelude.rs

This workflow is especially helpful when you’re building your own abstractions.