We could add such an ad-hoc-GLR-like algorithm to ECMA-262, but we would
lose the LR(1) (modulo ASI and lookahead restrictions, and the
conventional shift-reduce conflict resolution for dangling else)
validation I wrote about. That's considered unacceptable by TC39 members
who spoke up last time we talked about this.
But let's say we find a way forward on the validation front. Can't
assume this (paging Waldemar) but for now let's waive it.
Then the grammar would look something like this:
FunctionDeclaration:
function Identifier ( FormalParameterList_opt ) { FunctionBody }
Identifier ( FormalParameterList_opt ) [no LineTerminator here] {
FunctionBody }
ShortFunctionExpression:
Identifier_opt ( FormalParameterList_opt ) [no LineTerminator here]
{ FunctionBody }
Identifier_opt ( FormalParameterList_opt ) => InitialValue
Fat-arrow binds lexical |this| as in CoffeeScript and all right-thinking
languages :-P.
Change AssignmentExpression like so:
AssignmentExpression:
ConditionalExpression
LeftHandSideExpression = InitialValue
LeftHandSideExpression AssignmentOperator InitialValue
AssignmentExpressionNoIn:
ConditionalExpressionNoIn
LeftHandSideExpression = InitialValueNoIn
LeftHandSideExpression AssignmentOperator InitialValueNoIn
And change all uses of /AssignmentExpression/ outside of the
/Expression/ sub-grammar to /InitialValue/:
ElementList : // See 11.1.4
Elision_opt InitialValue
ElementList , Elision_opt InitialValue
PropertyAssignment : // See 11.1.5
PropertyName : InitialValue
ArgumentList : // See 11.2
InitialValue
ArgumentList , InitialValue
Initialiser : // See 12.2
= InitialValue
InitialiserNoIn : // See 12.2
= InitialValueNoIn
The cherry on top is the new InitialValue production:
InitialValue :
AssignmentExpression
ShortFunctionExpression
This avoids precedence inversion and restricts short function
expressions to appearing unparenthesized only where initial values may
occur today (in array and object literals, in argument lists, on the
right of assignment operators, and in variable initialisers).
/be
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