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DocumentationhackLearnExpression TreesSyntax Supported

Expression Trees: 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.hhi.

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<>)()
$x->bar(...) $v->visitMethodCall($position, $v->visitLocal($position, '$x'), 'bar', vec[...]) $x->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.

The visitor runtime only receives the method name for instance methods. For global functions and static methods, the visitor receives a function pointer, which contains more information.

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.
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