XXIIVV

Vera is a multiset rewriting system.

Vera is a multiset rewriting system designed to converse with computers, created by the same author as Modal. A program is made entirely of facts and rules:

|| The first character in a file assigns a spacer glyph.
|| A program is made of rules and facts.

|| Two spacers indicate the creation of facts
|| Facts are separated by commas, a matchbox, a log, paper

| A rule has a left side | And a right side.
| Facts on the left side | are replaced by facts in the right side

| a flame, a log | a warm fire
| a matchbox | a match, a match a box
| a match, paper | a flame

Spacer

The first character in a file assigns a spacer glyph to be used to indicate rules and facts, the pipe character | is most commonly used:

| this is a rule |
|| this is a fact, this is another fact

Facts

A fact is the name for an item in the bag, the bag is the collection of all existing facts. A line starting with two spacing characters(also called an empty rule) will add facts to the bag, facts are comma separated:

|| a fact, another fact
|| one more fact,
	yet another,
	a last fact

Whitespace only exists to separate the words in a fact, but are not significant when matching facts. These 3 facts are equal:

|| a new fact, a   new fact, a new   fact

Rules

A rule is made of a left and often a right side, and indicate facts to remove and replace in the bag.

| a fact, another fact | the result
| another rule | another result
| a rule may not have a right side |

Evaluation

In turn, each rule is matched against the existing facts found in the bag, starting from the first rule, when a rule's left-hand side is found in the bag, these facts are replaced by the rule's right-hand side, and matching is started again from the top. 

|flour, sugar, apples| apple cake
|apples, oranges, cherries| fruit salad
|fruit salad, apple cake| fruit cake

|| sugar, oranges, apples, apples, cherries, flour

Vera will match any amount found in the left-hand side, the following two rules are equivalent in Vera:

| an apple, hunger | a core
| an apple, an apple, an apple, hunger | a core

Comments

Vera will discard a rule when all the facts found in its left-hand side are not present in any rule's right-hand side or declared as facts.

| yellow | This is a comment.
| purple | This is not a comment.
| # | This is also a comment.
|| purple

Implementation

The language runtime can be implemented in about 200 lines.

cc vera.c -o vera view raw