Paren free

Paren-free Brendan Eich brendan@mozilla.orgSaturday, June 11, 2011

MotivationSaturday, June 11, 2011

HistorySaturday, June 11, 2011

History • JS derives from Java from C++ from C (via early C and B), from BCPL. BCPL had paren-free if, etc., heads disambiguated via the do reserved word to separate an expression consequent, avoiding ambiguity.Saturday, June 11, 2011

History • JS derives from Java from C++ from C (via early C and B), from BCPL. BCPL had paren-free if, etc., heads disambiguated via the do reserved word to separate an expression consequent, avoiding ambiguity. • Many JS style guides favor mandatory bracing of if consequents and other sub-statement bodies, in order to avoid dangling else bugs.Saturday, June 11, 2011

History • JS derives from Java from C++ from C (via early C and B), from BCPL. BCPL had paren-free if, etc., heads disambiguated via the do reserved word to separate an expression consequent, avoiding ambiguity. • Many JS style guides favor mandatory bracing of if consequents and other sub-statement bodies, in order to avoid dangling else bugs. • By comparison to nearby programming languages, JS syntax is “shifty” to write and noisy to read. Can we do better?Saturday, June 11, 2011

Examples if year > 2010 { syntax++ } for let i of iter { // i is fresh on each iteration frob(i) } while lo <= hi { let mid = (lo + hi) / 2 // binary search body goes here } ... return [i * i for i in range(n)] // array comprehensionSaturday, June 11, 2011

Examples, cont. • Must support: • but not mandate: if x < y { if x < y { } else if x < z { } else { } else if x < w { if x < z { } else { } else { } if x < w { } else { } } }Saturday, June 11, 2011

Syntax • IfStatement : if Expression SubStatement else SubStatement if Expression SubStatement if ( Expression ) OtherStatement else Statement if ( Expression ) OtherStatement • etc. for while, do-while, for, switch, catch • SubStatement : Block KeywordStatementSaturday, June 11, 2011

Syntax, cont. • KeywordStatement : • Statement : IfStatement Block IterationStatement KeywordStatement ContinueStatement OtherStatement BreakStatement ReturnStatement SwitchStatement • OtherStatement : ThrowStatement TryStatement EmptyStatement DebuggerStatement ExpressionStatement LabelledStatement VariableStatementSaturday, June 11, 2011

CompatibilitySaturday, June 11, 2011

Compatibility • if, while, do-while, and switch statements, where the head syntax is Expression, which may be over-parenthesized.Saturday, June 11, 2011

Compatibility • if, while, do-while, and switch statements, where the head syntax is Expression, which may be over-parenthesized. • try, catch, finally must be braced as in ES3 and ES5.Saturday, June 11, 2011

Compatibility • if, while, do-while, and switch statements, where the head syntax is Expression, which may be over-parenthesized. • try, catch, finally must be braced as in ES3 and ES5. • catch heads may be parenthesized or unparenthesized.Saturday, June 11, 2011

Compatibility • if, while, do-while, and switch statements, where the head syntax is Expression, which may be over-parenthesized. • try, catch, finally must be braced as in ES3 and ES5. • catch heads may be parenthesized or unparenthesized. • for heads may be parenthesized or unparenthesized.Saturday, June 11, 2011

Compatibility • if, while, do-while, and switch statements, where the head syntax is Expression, which may be over-parenthesized. • try, catch, finally must be braced as in ES3 and ES5. • catch heads may be parenthesized or unparenthesized. • for heads may be parenthesized or unparenthesized. • for-in/of heads may be parenthesized or unparenthesized.Saturday, June 11, 2011

Compatibility • if, while, do-while, and switch statements, where the head syntax is Expression, which may be over-parenthesized. • try, catch, finally must be braced as in ES3 and ES5. • catch heads may be parenthesized or unparenthesized. • for heads may be parenthesized or unparenthesized. • for-in/of heads may be parenthesized or unparenthesized. • for-of loop, comprehension, and generator expression new semantics (next slide).Saturday, June 11, 2011

for-in & for-ofSaturday, June 11, 2011

for-in & for-of • for k in o iterates over keys not values for all objects o.Saturday, June 11, 2011

for-in & for-of • for k in o iterates over keys not values for all objects o. • for k of {p:1, q:2} iterates over 1, 2.Saturday, June 11, 2011

for-in & for-of • for k in o iterates over keys not values for all objects o. • for k of {p:1, q:2} iterates over 1, 2. • for v of [3, 4, 5] iterates over 3, 4, 5.Saturday, June 11, 2011

for-in & for-of • for k in o iterates over keys not values for all objects o. • for k of {p:1, q:2} iterates over 1, 2. • for v of [3, 4, 5] iterates over 3, 4, 5. • for k of keys(o) iterates over keys in o.Saturday, June 11, 2011

for-in & for-of • for k in o iterates over keys not values for all objects o. • for k of {p:1, q:2} iterates over 1, 2. • for v of [3, 4, 5] iterates over 3, 4, 5. • for k of keys(o) iterates over keys in o. • for v of values(o) iterates over values in o.Saturday, June 11, 2011

for-in & for-of • for k in o iterates over keys not values for all objects o. • for k of {p:1, q:2} iterates over 1, 2. • for v of [3, 4, 5] iterates over 3, 4, 5. • for k of keys(o) iterates over keys in o. • for v of values(o) iterates over values in o. • for [k, v] of items(o) iterates over key/value pairs.Saturday, June 11, 2011

for-in & for-of • for k in o iterates over keys not values for all objects o. • for k of {p:1, q:2} iterates over 1, 2. • for v of [3, 4, 5] iterates over 3, 4, 5. • for k of keys(o) iterates over keys in o. • for v of values(o) iterates over values in o. • for [k, v] of items(o) iterates over key/value pairs. • for x of proxy iterates using the handler iterate trap. (http://wiki.ecmascript.org/doku.php?id=harmony:iterators)Saturday, June 11, 2011

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