Making Languages

Part 6: Compilation

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Edit:

Anthony Ferrara points out that my wording is unclear here. Compilers convert code from one programming language to another.

It’s much easier to convert code from a high-level language to a fast, low-level language than it is to make that language. Perhaps you are tied into using a language like PHP, but you’d like to use a different syntax. You can do just that!

Compiling

Most of the work was done during parsing and interpretation. We just need to add a few render() methods to our constructs:

/**
 * @param array $context
 *
 * @return string
 */
public function render(array $context)
{
  return '$' . $this->identity .' = '.
    $this->value->render($context);
}

This is from Assignment.php

/**
 * @param array $context
 *
 * @return string
 */
public function render(array &$context)
{
  return (string) $this->value;
}

This is from Number.php

/**
 * @param array $context
 *
 * @return string
 */
public function render(array &$context)
{
  return $this->left->render($context) . ' + ' .
    $this->right->render($context);
}

This is from Addition.php

That’s it! We no longer have to worry about calculating values — we’ve moved on to rendering valid PHP from our Abstract Syntax Tree:

$context = [];
 
$nodes = [
  new Assignment('foo',
    new Addition(
      new Addition(
        new Number(1),
        new Number(2)
      ),
      new Number(3)
    )
  ),
  new Assignment('bar',
    new Number(5)
  )
];
 
foreach ($nodes as $node) {
  print $node->render($context) . ";\n";
}
 
/*
 * $foo = 1 + 2 + 3;
 * $bar = 5;
 */

Context

Even though we don’t use the $context; it will be useful later on. Context can store type data, as well as value data. This means we can type-check our code, even if the target language doesn’t support static typing.

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