# Invoking the FhirPath Compiler directly

The FhirPath compiler is just another public class in the `Hl7.FhirPath` namespace. It has a constructor which takes an argument of type `SymbolTable` - the key
to full control over the installed dialect:

```csharp
var symbolTable = new SymbolTable()
      .AddStandardFP()
      .AddFhirExtensions();
var newCompiler = new FhirPathCompiler(symbolTable);
```

You can now use the compiler to:

* `Compile()` an expression to a ready-to-execute delegate (called `CompiledExpression`)
* `Parse()` an expression to an abstract symbol tree, for display or debugging purposes

Invoking the `CompiledExpression` is equivalent to using the `Select()` function described above. The other functions, like `IsBoolean`
are also available (as extension methods).

Note that the built-in extension methods (`Select()`, `Scalar()`, etc.) don't recompile the expression on every call: internally they use a
`FhirPathCompilerCache`, which pairs a configured `FhirPathCompiler` with a cache of already-compiled expressions (by default holding up to 500 entries).
If you invoke your own `FhirPathCompiler` directly, you bypass this cache — so for expressions you evaluate repeatedly, either cache the resulting
`CompiledExpression` yourself or create your own `FhirPathCompilerCache` (its constructor takes an optional `FhirPathCompiler` and cache size).