If let statement

The if..let statement may be used for pattern matching on algebraic data types.

if (let Plus(10, right) = exp) {
    //
}

It may also be used for extracting a non-nullable variable from a test expression, without using a matching pattern; that is, a regular identifier binding.

if (let node = parseAtom()) {
    //
}

if..let can also be useful for, say, iterator results.

if (let [x!,false] = characters.next()) {
    //
} else {
    break;
}

Syntax

IfLetStatement^abbrev :
if ( IfLetVariable ) Substatement^abbrev
if ( IfLetVariable ) Substatement^noShortIf else Substatement^abbrev

IfLetStatement^full :
if ( IfLetVariable ) Substatement^full
if ( IfLetVariable ) Substatement^noShortIf else Substatement^full

IfLetStatement^noShortIf :
if ( IfLetVariable ) Substatement^noShortIf else Substatement^noShortIf

IfLetVariable :
VariableDefinitionKind VariableBinding^allowIn