Exercises

A few exercises to practice the evaluation model. Each has a worked solution you can run in this engine (paste it into a .metta file and run metta run file.metta, or pass it to runProgram). Try them before peeking.

A reminder on the list encoding used below: the empty list is () and a cons cell is (:: head tail), so [A, B, C] is (:: A (:: B (:: C ()))).

1. Store facts, then query them

Store that Ada likes coffee and chocolate and Turing likes tea, then ask what Ada likes.

(Likes Ada Coffee)
(Likes Ada Chocolate)
(Likes Turing Tea)

!(match &self (Likes Ada $thing) $thing)   ; Coffee and Chocolate

match searches the stored atoms (not sub-expressions) for the pattern and returns the template $thing for each match.

2. inc, dec, square

Define (inc $x), (dec $x), and (square $x) with grounded arithmetic.

(= (inc $x) (+ $x 1))
(= (dec $x) (- $x 1))
(= (square $x) (* $x $x))

!(inc 41)      ; 42
!(dec 10)      ; 9
!(square 12)   ; 144

3. classify by sign

Define (classify $x) returning Positive, Negative, or Zero. Nested if does it:

(= (classify $x)
   (if (> $x 0) Positive
       (if (< $x 0) Negative Zero)))

!(classify 7)    ; Positive
!(classify -3)   ; Negative
!(classify 0)    ; Zero

4. factorial

Grounded numbers cannot be deconstructed by pattern matching, so use if for the base case:

(= (fact $n) (if (> $n 0) (* $n (fact (- $n 1))) 1))

!(fact 0)   ; 1
!(fact 5)   ; 120
!(fact 7)   ; 5040

5. length of a list

Two rules, a base case and a recursive case:

(= (length ()) 0)
(= (length (:: $x $xs)) (+ 1 (length $xs)))

!(length (:: A (:: B (:: C ()))))   ; 3

Doing the same from TypeScript

Every exercise above can be written with the typed eDSL instead of source strings. Factorial, for example:

import { mettaDB, names, vars, If, gt, mul, sub } from "@mettascript/edsl";

const db = mettaDB();
const { fact } = names();
const { x } = vars();
db.rule(fact(x), If(gt(x, 0), mul(x, fact(sub(x, 1))), 1));
db.evalJs(fact(5)); // [120]