The Workflow

Both evaluate() and compile() need to do some things before each can provide the return values. For evaluate(), this overhead is even bigger.

Both methods need to tokenize and parse the expression. This is done by the parse() method. It returns a ParsedExpression. Now, the compile() method just returns the string conversion of this object. The evaluate() method needs to loop through the "nodes" (pieces of an expression saved in the ParsedExpression) and evaluate them on the fly.

To save time, the ExpressionLanguage caches the ParsedExpression so it can skip the tokenize and parse steps with duplicate expressions. The caching is done by a ParserCacheInterface instance (by default, it uses an ArrayParserCache). You can customize this by creating a custom ParserCache and injecting this in the object using the constructor:

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use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
use Acme\ExpressionLanguage\ParserCache\MyDatabaseParserCache;

$cache = new MyDatabaseParserCache(...);
$language = new ExpressionLanguage($cache);

Using Parsed and Serialized Expressions

Both evaluate() and compile() can handle ParsedExpression and SerializedParsedExpression:

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use Symfony\Component\ExpressionLanguage\ParsedExpression;
// ...

$expression = new ParsedExpression($language->parse('1 + 4'));

echo $language->evaluate($expression); // prints 5

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use Symfony\Component\ExpressionLanguage\SerializedParsedExpression;
// ...

$expression = new SerializedParsedExpression(
    serialize($language->parse('1 + 4'))
);

echo $language->evaluate($expression); // prints 5