Syntax Supported

Expression tree DSLs can use most of the expression and statement syntax in Hack.

For a full list of supported syntax, see also expr_tree.php.

Literals

Example	Visitor Runtime	Typing
true	$v->visitBool($position, true)	MyDsl::boolType()
1	$v->visitInt($position, 1)	MyDsl::intType()
1.23	$v->visitFloat($position, 1.23)	MyDsl::floatType()
"foo"	$v->visitString($position, "foo")	MyDsl::stringType()
null	$v->visitString($position)	MyDsl::nullType()
N/A	N/A	MyDsl::voidType()

Binary Operators

Operator names are based on the appearance of the symbol, as different DSLs may choose different semantics for an operator.

Example	Visitor Runtime	Typing
$x + $y	$v->visitBinop($position, ..., '__plus', ...)	__plus method on $x
$x - $y	$v->visitBinop($position, ..., '__minus', ...)	__minus method on $x
$x * $y	$v->visitBinop($position, ..., '__star', ...)	__star method on $x
$x / $y	$v->visitBinop($position, ..., '__slash', ...)	__slash method on $x
$x % $y	$v->visitBinop($position, ..., '__percent', ...)	__percent method on $x
$x && $y	$v->visitBinop($position, ..., '__ampamp', ...)	__ampamp method on $x
$x || $y	$v->visitBinop($position, ..., '__barbar', ...)	__barbar method on $x
$x < $y	$v->visitBinop($position, ..., '__lessThan', ...)	__lessThan method on $x
$x <= $y	$v->visitBinop($position, ..., '__lessThanEqual', ...)	__lessThanEqual method on $x
$x > $y	$v->visitBinop($position, ..., '__greaterThan', ...)	__greaterThan method on $x
$x >= $y	$v->visitBinop($position, ..., '__greaterThanEqual', ...)	__greaterThanEqual method on $x
$x === $y	$v->visitBinop($position, ..., '__tripleEquals', ...)	__tripleEquals method on $x
$x !== $y	$v->visitBinop($position, ..., '__notTripleEquals', ...)	__notTripleEquals method on $x
$x . $y	$v->visitBinop($position, ..., '__dot', ...)	__dot method on $x
$x & $y	$v->visitBinop($position, ..., '__amp', ...)	__amp method on $x
$x | $y	$v->visitBinop($position, ..., '__bar', ...)	__bar method on $x
$x ^ $y	$v->visitBinop($position, ..., '__caret', ...)	__caret method on $x
$x << $y	$v->visitBinop($position, ..., '__lessThanLessThan', ...)	__lessThanLessThan method on $x
$x >> $y	$v->visitBinop($position, ..., '__greaterThanGreaterThan', ...)	__greaterThanGreaterThan method on $x

Ternary Operators

Example	Visitor Runtime	Typing
$x ? $y : $z	$v->visitTernary($position, ..., ..., ...)	$x->__bool() ? $y : $z

Unary Operators

Example	Visitor Runtime	Typing
!$x	$v->visitUnop($position, ..., '__exclamationMark')	__exclamationMark method on $x
-$x	$v->visitUnop($position, ..., '__negate')	__negate method on $x
~$x	$v->visitUnop($position, ..., '__tilde')	__tilde method on $x
$x++	$v->visitUnop($position, ..., '__postfixPlusPlus')	__postfixPlusPlus method on $x
$x--	$v->visitUnop($position, ..., '__postfixMinusMinus')	__postfixMinusMinus method on $x

Local Variables

Example	Visitor Runtime	Typing
$x	$v->visitLocal($position, '$x')	Same as normal Hack

You can see here that the visitor runtime does not know the type of $x, it just sees a call to visitLocal with the variable name as a string '$x'.

Note that expression trees do not allow free variables. You can only use local variables that have been previously assigned or introduced with a lambda.

Lambdas

Example	Visitor Runtime	Typing
(Foo $x) ==> $y	$v->visitLambda($position, vec['$x'], vec[...])	Same as normal Hack

Note that the visitor runtime does not see the type of $x.

Statements

Example	Visitor Runtime	Typing
$x = $y;	$v->visitAssign($position, ..., ...)	Same as normal Hack
return $x;	$v->visitReturn($position, ...)	Same as normal Hack
return;	$v->visitReturn($position, null)	Same as normal Hack
if (...) {...} else {...}	$v->visitIf($position, ..., ..., ...)	if (...->__bool()) {...} else {...}
while (...) {...}	$v->visitWhile($position, ..., vec[...])	while (...->__bool()) {...}
for (...; ...; ...) {...}	$v->visitFor($position, vec[...], ..., vec[...], vec[...])	for (...; ...->__bool(); ...) {...}
break;	$v->visitBreak($position)	Same as normal Hack
continue;	$v->visitContinue($position)	Same as normal Hack

Calls

Example	Visitor Runtime	Typing
foo(...)	$v->visitCall($position, $v->visitGlobalFunction($position, foo<>), vec[...])	MyDsl::symbolType(foo<>)()
Foo::bar(...)	$v->visitCall($position, $v->visitStaticMethod($position, Foo::bar<>), vec[...])	MyDsl::symbolType(Foo::bar<>)()

Note that the function or method must have a Hack definition, so the typechecker can verify that it's being called with appropriate arguments.

XHP Literals

Example	Visitor Runtime	Typing
<foo ...>...</foo>	$v->visitXhp($position, :foo::class, dict[...], vec[...])	<foo ...>...</foo>

Property Access

Example	Visitor Runtime	Typing
(...)->foo	$v->visitPropertyAccess($position, ..., 'foo')	(...)->foo

Splicing

Example	Visitor Runtime	Typing
${$x}	$v->splice($position, 'key0', $x)	Extract the inferred type out of the third type argument of Spliceable

Unsupported Features

Expression trees may only contain expressions between the backticks.

MyDsl`1`; // OK: 1 is an expression
MyDsl`while(true) {}`; // Bad: statement
MyDsl`() ==> { while true() {} }`; // OK: statements are allowed in lambdas
MyDsl`class Foo {}`; // Bad: top-level declaration.

Expression Tree Blocks

There are times when it is desirable to include statements as part of an expression tree. One way to accomplish this is to create a lambda that is immediately invoked.

<<file:__EnableUnstableFeatures('expression_trees')>>

function example1(): void {
  $num = ExampleDsl`1 + 1`;
  ExampleDsl`() ==> {
    $n = ${$num};
    return $n + $n;
  }()`;
}

This can be rewritten using expression tree blocks, eliminating the need to create a lambda and invoke it.

<<file:__EnableUnstableFeatures('expression_trees')>>

function example2(): void {
  $num = ExampleDsl`1 + 1`;
  ExampleDsl`{
    $n = ${$num};
    return $n + $n;
  }`;
}

Note that expression tree blocks are syntactic sugar. It will be expanded to the longer form for both type checking and the visitor runtime.