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description Pre-compile and cache FHIRPath expressions for repeated evaluation.
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Compilation, Caching & Performance

Parsing a FHIRPath expression into an AST is the costly step. To avoid repeating it, expressions can be pre-compiled once and evaluated many times, and the service caches compiled expressions automatically.

Compilation

compile() parses an expression and returns a CompiledExpression (Ardenexal\FHIRTools\Component\FHIRPath\Service\CompiledExpression) holding the pre-parsed AST. Evaluating it skips lexing and parsing entirely:

// Compile once, evaluate many times
$compiled = $service->compile('name.where(use = "official").given.first()');

foreach ($patients as $patient) {
    $result = $compiled->evaluate($patient);
}

A CompiledExpression also exposes getExpression(), getAst(), and __toString().

Caching

evaluate() and compile() both route through an internal cache, so even without calling compile() explicitly, repeated evaluation of the same expression string reuses the parsed AST.

The default cache is InMemoryExpressionCache (Ardenexal\FHIRTools\Component\FHIRPath\Cache\InMemoryExpressionCache), which keys compiled expressions by their source string.

{% hint style="info" %} The cache holds up to 100 expressions by default and evicts the least recently used (LRU) entry when full. Construct the cache with a different maxSize to tune this. {% endhint %}

Custom cache

Inject any ExpressionCacheInterface implementation via the constructor:

use Ardenexal\FHIRTools\Component\FHIRPath\Cache\InMemoryExpressionCache;
use Ardenexal\FHIRTools\Component\FHIRPath\Service\FHIRPathService;

$service = new FHIRPathService(new InMemoryExpressionCache(maxSize: 500));

Cache statistics and control

$stats = $service->getCacheStats();
// ['hits' => 10, 'misses' => 2, 'size' => 5]

$service->clearCache();          // empty the cache and reset counters
$cache = $service->getCache();   // the underlying ExpressionCacheInterface

The in-memory cache additionally exposes getHitRate() (0–100) and getMaxSize().

Tips

  • Compile expressions used in hot loops once, outside the loop.
  • Reuse a single FHIRPathService instance so its cache survives across calls.
  • For large batches that exceed the default 100-entry cache, raise maxSize to keep all expressions resident and avoid re-parsing on eviction.