Main concepts
This Learn MeTTa track follows the official tutorials at metta-lang.dev. It teaches the same concepts (main concepts, basic evaluation, recursion, nondeterminism) adapted to run in this engine. For the canonical material, read the originals there.
A MeTTa program lives inside a space: a database of atoms with a query engine over it. A program holds both knowledge (facts) and code (rules), and because code is just atoms too, a program can inspect and rewrite itself. If you have used Prolog, the idea of a program as a queryable knowledge base will feel familiar; MeTTa takes it further and makes everything, including the rules, ordinary data.
The four kinds of atoms
Everything in a space is an atom, and there are exactly four kinds.
A Symbol is a name standing for some concept: A, f, parent, known?. Two symbols with the same name are the same concept. Almost any string can be a symbol.
An Expression groups other atoms in parentheses, Scheme-style: (f A), (parent Tom Bob), (implies (human Socrates) (mortal Socrates)). Expressions nest freely, so an expression is really a tree.
A Variable is written with a leading $: $x, $_, $thing. Variables turn an expression into a pattern. A pattern like (parent $x Bob) gets its meaning when it is matched against other atoms, binding $x to whatever makes the match succeed.
A Grounded atom wraps sub-symbolic data: a concrete value, a collection, or an operation. The numbers and arithmetic you use are grounded: in (+ 1 2), the + is a grounded operation and 1 and 2 are grounded values. Grounded atoms are how MeTTa reaches into the host language, and in MeTTaScript that host is TypeScript, so a grounded atom can hold any TypeScript value.
Here is that nesting made literal. This is one atom drawn as a tree: each node is a symbol, a variable, or a value, and the parentheses become the parent-child links. Drag to pan and use the zoom controls to fit; the full editor lives on the visual editor page.
Facts versus evaluation
A MeTTa script is read one atom at a time. An atom on its own is added to the space. An atom prefixed with ! is evaluated immediately, and its result is printed rather than stored. Comments start with ;.
The first line stores a fact. The second computes 3 and shows it. Nothing about (Likes Alice Pizza) is evaluated; it just sits in the space as knowledge you can query later.
Special symbols
Three symbols are not grounded operations but are still treated specially by the interpreter, because they shape how a program is evaluated and typed.
The equality symbol = defines evaluation rules. Read (= lhs rhs) as "lhs can be reduced to rhs". This is how you define what amounts to a function.
The colon symbol : declares a type, as in (: Socrates Human).
The arrow symbol -> builds a function type, as in (: greet (-> Symbol Symbol)).
You will use = constantly, and meet : and -> again when we get to types. They are ordinary symbols, but the interpreter knows what to do with them.
Types and metatypes
MeTTa has optional typing, close to gradual dependent types. Untyped expressions have the type %Undefined%; other types are just symbols and expressions you declare. On top of that sit four metatypes, Symbol, Variable, Grounded, and Expression, which let a program reason about the shape of atoms themselves. The Types page covers all of this; for now it is enough to know they are optional and that you can write plenty of MeTTa without them.
Next, let us actually evaluate things: Basic evaluation.