Interpreter Pattern

Also Known As

TL;DR

Explanation

Terminal expressions (values) and non-terminal expressions (AND, OR, NOT) form a composite tree. Parse input to build the tree; interpret evaluates it. PHP use cases: search query parsers, business rule engines. For complex grammars use a proper parser (PHP-Parser) rather than hand-rolling the Interpreter pattern.

Common Misconception

Why It Matters

Common Mistakes

• Using for complex grammars — use proper parser
• Not caching parsed expressions
• No error handling in terminals

Code Examples

function evalRule(string $rule, Order $o): bool {
    if (preg_match('/total > (\d+)/',$rule,$m)) return $o->total > $m[1];
    // Cannot handle AND, OR
}

interface Expression { public function interpret(Context $ctx): bool; }
class GreaterThan implements Expression {
    public function __construct(private string $f, private float $v) {}
    public function interpret(Context $ctx): bool { return $ctx->get($this->f) > $this->v; }
}
class AndExpr implements Expression {
    public function __construct(private Expression $l, private Expression $r) {}
    public function interpret(Context $ctx): bool { return $this->l->interpret($ctx) && $this->r->interpret($ctx); }
}

References

• ↗ https://refactoring.guru/design-patterns/interpreter

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