# Building the parser


## Install node

We need to use node:

    apt-get install node npm

Inside `stack/stack/maximaparser` (or anywhere else), and not as root

    npm install pegjs 
    npm install phpegjs ts-pegjs

## Compile the parser

To compile the parser

    node gen.js

This should return `null null null` and create new parsers for STACK.

## Adding rules to the parser

Identical changes need to be made in both

1. `stack/maximaparser/MP_classes.php`
2. `stack/maximaparser/autogen/parser-grammar.pegjs`


## Naming parser rules

We give each parser rule a name, numbered in the approximate appropriate order in which they need to be applied.

* 0-10 the mandatory stars for "f(x)(y) => f(x)*(y)" and syntactical candy like the logarithms
* 10-20 rules forbidding certain usages.
* 20-30 identification of typical syntax mistakes like "sin^2(x)".
* 30-40 rules forbidding certain usages, like calling functions at all.
* 40-50 various rules adding stars, insert stars in function calls and split variables and whatnot.
* 60-70 rules forbidding certain usages, e.g. forbid floats.
* 80-90 logic to check for things that have been done and deciding whether what was done was ok i.e. stars were inserted but not allowed in "syntax=true".

## Debugging the parser rules

    echo "\n" . $ast->debugPrint($ast->toString() . '     ') . "\n";
    while ($ast->callbackRecurse($process, true) !== true) {
        echo "\n" . $ast->debugPrint($ast->toString() . '     ') . "\n";
    }
    echo "\n" . $ast->debugPrint($ast->toString() . '     ') . "\n";

# Nouns

There are two separate classes of expressions which need to be protected as "nouns".

1. The Maxima Boolean functions do not respect `simp:false`.  So, we have parallel operators such as `A nounand B`.  These should always be used when connecting to Maxima.  Evaluation/simplification of Boolean expressions such as `true and false` is done on the Maxima side.  Teachers and students should use `and`, etc. and these are always translated into an evaluation form.
2. With `simp:false` we still have evaluation of expressions such as `diff(x^3,x)` to `3*x^2`.  To protect student's answers ("Your last answer was..") we  have parallel noun forms such as `noundiff` and `nounint`.  Note, some of these also change the display of expressions.
3. Maxima uses the apostophie to create noun forms, e.g. `'diff(x^3,x)`.  From STACK 4.3, teachers are able to use this.