Expression Trees: Defining Dsls

Adding a DSL to Hack requires you to define a new visitor.

This page will demonstrate defining a DSL supporting integer arithmetic:

$e = MyDsl`1 + 2`;

DSLs are opt-in

Hack only allows expression tree syntax usage for approved DSLs. These are specified in .hhconfig.

allowed_expression_tree_visitors = MyDsl, OtherDsl

If you just want to test your DSL, you can enable all expression syntax with the __EnableUnstableFeatures file attribute.


function foo(): void {
  $e = MyDsl`1 + 2`;

Representing DSL Expressions

Our DSL needs a data type to represent expressions written by the user. We'll define a simple abstract syntax tree (AST).

abstract class MyDslAst {}

class MyDslAstBinOp extends MyDslAst {
  public function __construct(
    public MyDslAst $lhs,
    public string $operator,
    public MyDslAst $rhs,
  ) {}

class MyDslAstInt extends MyDslAst {
  public function __construct(public int $value) {}

A DSL For Integer Literals

Hack converts backtick syntax into method calls.

// The user writes backtick syntax.
$e = MyDsl`1`;

// The runtime sees a lambda calling methods on MyDSL (simplified)
(MyDsl $v) ==> $v->visitInt(null, 1);

Our basic visitor looks like this.

// The runtime will pass file and line position in ExprPos.
type ExprPos = shape(...);

class MyDsl {
  // The visitor is passed the literal value, so 1 in our example.
  public function visitInt(?ExprPos $_pos, int $value): MyDslAst {
    return new MyDslAstInt($value);

Adding Operators

// User syntax.
$e = MyDsl`1 + 2`;

// Runtime (simplified)
(MyDsl $v) ==>
    $v->visitInt(null, 1),
    $v->visitInt(null, 2)

You can see that $v-visitBinop() receives the return value of $v->visitInt(). This allows the visitor to construct more complex ASTs.

The visitFoo methods are always unityped. They receive ASTs without type information, and return an untyped AST. Typechecking happens separately.

We can support binary operators by adding visitBinop to our visitor.

type ExprPos = shape(...);

class MyDsl {
  public function visitBinop(
    ?ExprPos $_pos,
    MyDslAst $lhs,
    string $operator,
    MyDslAst $rhs,
  ): MyDslAst {
    return new MyDslAstBinOp($lhs, $operator, $rhs);

  public function visitInt(?ExprPos $_pos, int $value): MyDslAst {
    return new MyDslAstInt($value);

The DSL Builder

The visitor closure isn't enough. We want to type check the DSL expression, and we might want to do additional work before we execute the closure.

// The user writes backtick syntax.
$e = MyDsl`1`;

// Runtime (actual). The first two arguments are extra position information
// and function metadata which aren't used in this tutorial.
MyDsl::makeTree<MyDslInt>(null, shape(), (MyDsl $v) ==> $v->visitInt(null, 1))

We call makeTree on our visitor class, providing the visitor closure and some additional metadata. The type checker also sees the TInfer type, which is MyDslInt in this example (discussed below).

class MyDsl {
  public static function makeTree<<<__Explicit>> TInfer>(
    ?ExprPos $pos,
    mixed $_metadata,
    (function(MyDsl): MyDslAst) $visit_expr,
  ): MyDslExprTree<TInfer> {
    return new MyDslExprTree($pos, $visit_expr);
  // ... all the visitFoo methods here


The builder method makeTree takes a nullable position argument as its first argument, as do all of the visit... methods on the Visitor class. At runtime, all of the methods will receive a shape value with the following type:

  'path' => string,
  'start_line' => int,
  'start_column' => int,
  'end_line' => int,
  'end_column' => int,

DSLs can choose to use this information to report errors with positional information about the source code back to the user.


The second argument to the makeTree method is a shape containing parts of the expression tree that may be useful to have references to without processing the entirety of the $visit_expr. The shape has the type:

  'splices' => dict<string, mixed>,
  'functions' => vec<mixed>,
  'static_methods' => vec<mixed>,
) $metadata

For the functions and static_methods fields, the vec contains the function pointers to any global functions or static method referenced within the expression tree. The same function pointers are passed to the individual calls of visitGlobalFunction and visitStaticMethod.

The splices field contains a dictionary of string keys and the values the splices are evaluated to. The string keys are generated by the runtime and correspond to the string key and value passed to each individual splice Visitor method call.

Spliceable Types

Expression tree values implement Spliceable, a typed value that can be visited or spliced into another Spliceable. Here's the definition for the Spliceable interface.

 * Spliceable is this base type for all expression tree visitors.
 * A visitor is a class with a visit method. This is extremely generic, so
 * visitors can choose what they want to construct.
 * Typically, you'll use a concrete type rather than this interface. For
 * example:
 *     $e = EtDemo`123`;
 * This has type `Spliceable<EtDemoVisitor, EtDemoAst, EtDemoInt>`.
 * TVisitor: The class with the `visit` method that constructs the value.
 * TResult: The type we get back when running the visitor.
 * TInfer: The inferred type of the expression, used only for type checking.
interface Spliceable<TVisitor, TResult, +TInfer> {
  public function visit(TVisitor $visitor): TResult;

The MyDslExprTree class implements Spliceable, and calls the visitor closure ('builder') when visit is called.

class MyDslExprTree<+T> implements Spliceable<MyDsl, MyDslAst, T> {
  public function __construct(
    public ?ExprPos $pos,
    private (function(MyDsl): MyDslAst) $builder,
  ) {}

  public function visit(MyDsl $v): MyDslAst {
    return ($this->builder)($v);

DSL Types

// Inferred type: MyDslExprTree<MyDslInt>
$e = MyDsl`1`;

Hack needs to know what types our DSL uses. The visitor specifies the type of literals using fooType stub methods.

class MyDsl {
  public static function intType(): MyDslInt {
    // Only used for type checking, so we don't need an implementation here.
    throw new \Exception();
  // ...

For each type in our DSL, we define an associated type. It's usually easier to use interfaces for these types, as they're only used for type checking.

interface MyDslNonnull {}

interface MyDslInt extends MyDslNonnull {
  public function __plus(MyDslInt $_): MyDslInt;
  public function __minus(MyDslInt $_): MyDslInt;

This allows integers to support the + and - operators in our DSL, and they return DSL integers.