Improved lookahead for arrow functions.

Fixes issue #34.

Specifically:
    * We no longer automatically assume "()" is a function expression. It must be followed by an arrow, colon, or curly brace.
    * If an arrow is missing following a signature, but a curly brace is present, we assume the user forgot the arrow and try to parse the body anyway.

src/compiler/parser.ts

@@ -1445,6 +1445,7 @@ module ts {
         function tryParseParenthesizedArrowFunctionExpression(): Expression {
             var pos = getNodePos();
 
+            // Whether we are certain that we should parse an arrow expression.
             var triState = isParenthesizedArrowFunctionExpression();
 
             // It is not a parenthesized arrow function.
@@ -1457,11 +1458,12 @@ module ts {
                 var sig = parseSignature(SyntaxKind.CallSignature, SyntaxKind.ColonToken);
 
                 // If we have an arrow, then try to parse the body.
-                if (parseExpected(SyntaxKind.EqualsGreaterThanToken)) {
-                    return parseArrowExpressionTail(pos, sig, /*noIn:*/ false);
+                // Even if not, try to parse if we have an opening brace, just in case we're in an error state.
+                if (parseExpected(SyntaxKind.EqualsGreaterThanToken) || token === SyntaxKind.OpenBraceToken) {
+                    return parseArrowExpressionTail(pos, sig, /* noIn: */ false);
                 }
-                // If not, we're probably better off bailing out and returning a bogus function expression.
                 else {
+                    // If not, we're probably better off bailing out and returning a bogus function expression.
                     return makeFunctionExpression(SyntaxKind.ArrowFunction, pos, /* name */ undefined, sig, createMissingNode());
                 }
             }
@@ -1477,29 +1479,44 @@ module ts {
             }
         }
 
-        //  True        -> There is definitely a parenthesized arrow function here. 
-        //  False       -> There is definitely *not* a parenthesized arrow function here.
+        //  True        -> We definitely expect a parenthesized arrow function here.
+        //  False       -> There *cannot* be a parenthesized arrow function here.
         //  Unknown     -> There *might* be a parenthesized arrow function here.
-        //                  Speculatively look ahead to be sure.
+        //                 Speculatively look ahead to be sure, and rollback if not.
         function isParenthesizedArrowFunctionExpression(): Tristate {
             if (token === SyntaxKind.OpenParenToken || token === SyntaxKind.LessThanToken) {
                 return lookAhead(() => {
                     var first = token;
-                    nextToken();
+                    var second = nextToken();
+
                     if (first === SyntaxKind.OpenParenToken) {
-                        if (token === SyntaxKind.CloseParenToken || token === SyntaxKind.DotDotDotToken) {
-                            // Simple cases.  if we see ()   or   (...   then presume that presume 
-                            // that this must be an arrow function.  Note, this may be too aggressive
-                            // for the "()" case.  It's not uncommon for this to appear while editing 
-                            // code.  We should look to see if there's actually a => before proceeding.
+                        if (second === SyntaxKind.CloseParenToken) {
+                            // Simple cases: "() =>", "(): "  "() {".
+                            // This is an arrow function with no parameters.
+                            // The last one is not actually an arrow function,
+                            // but this is probably what the user intended.
+                            var third = nextToken();
+                            switch (third) {
+                                case SyntaxKind.EqualsGreaterThanToken:
+                                case SyntaxKind.ColonToken:
+                                case SyntaxKind.OpenBraceToken:
+                                    return Tristate.True;
+                                default:
+                                    return Tristate.False;
+                            }
+                        }
+
+                        // Simple case: "(..."
+                        // This is an arrow function with a rest parameter.
+                        if (second === SyntaxKind.DotDotDotToken) {
                             return Tristate.True;
                         }
 
+                        // We had "(" not followed by an identifier. This definitely doesn't
+                        // look like a lambda. Note: we could be a little more lenient and allow
+                        // "(public" or "(private". These would not ever actually be allowed,
+                        // but we could provide a good error message instead of bailing out.
                         if (!isIdentifier()) {
-                            // We had "(" not followed by an identifier.  This definitely doesn't 
-                            // look like a lambda.  Note: we could be a little more lenient and allow
-                            // (public   or  (private.  These would not ever actually be allowed,
-                            // but we could provide a good error message instead of bailing out.
                             return Tristate.False;
                         }
 

tests/baselines/reference/arrowFunctionsMissingTokens.errors.txt

@@ -0,0 +1,145 @@
+==== tests/cases/compiler/arrowFunctionsMissingTokens.ts (39 errors) ====
+
+    module missingArrowsWithCurly {
+        var a = () { };
+                   ~
+!!! '=>' expected.
+
+        var b = (): void { }
+                         ~
+!!! '=>' expected.
+
+        var c = (x) { };
+                    ~
+!!! ',' expected.
+                 ~
+!!! Cannot find name 'x'.
+
+        var d = (x: number, y: string) { };
+                  ~
+!!! ')' expected.
+                                     ~
+!!! ',' expected.
+                                       ~
+!!! Variable declaration expected.
+                 ~
+!!! Cannot find name 'x'.
+
+        var e = (x: number, y: string): void { };
+                  ~
+!!! ')' expected.
+                                     ~
+!!! ',' expected.
+                                      ~
+!!! Variable declaration expected.
+                                        ~~~~
+!!! Variable declaration expected.
+                 ~
+!!! Cannot find name 'x'.
+    }
+
+    module missingCurliesWithArrow {
+        module withStatement {
+            var a = () => var k = 10;};
+                          ~~~
+!!! '{' expected.
+
+            var b = (): void => var k = 10;}
+                                ~~~
+!!! '{' expected.
+
+            var c = (x) => var k = 10;};
+                           ~~~
+!!! '{' expected.
+
+            var d = (x: number, y: string) => var k = 10;};
+                                              ~~~
+!!! '{' expected.
+
+            var e = (x: number, y: string): void => var k = 10;};
+                                                    ~~~
+!!! '{' expected.
+
+            var f = () => var k = 10;}
+                          ~~~
+!!! '{' expected.
+        }
+
+        module withoutStatement {
+            var a = () => };
+                          ~
+!!! Expression expected.
+
+            var b = (): void => }
+                                ~
+!!! Expression expected.
+
+            var c = (x) => };
+                           ~
+!!! Expression expected.
+
+            var d = (x: number, y: string) => };
+                                              ~
+!!! Expression expected.
+
+            var e = (x: number, y: string): void => };
+                                                    ~
+!!! Expression expected.
+
+            var f = () => }
+                          ~
+!!! Expression expected.
+        }
+        ~
+!!! Declaration or statement expected.
+    }
+    ~
+!!! Declaration or statement expected.
+
+    module ceci_nEst_pas_une_arrow_function {
+        var a = ();
+                 ~
+!!! Expression expected.
+
+        var b = (): void;
+                        ~
+!!! '=>' expected.
+
+        var c = (x);
+                 ~
+!!! Cannot find name 'x'.
+
+        var d = (x: number, y: string);
+                  ~
+!!! ')' expected.
+                                     ~
+!!! ',' expected.
+                 ~
+!!! Cannot find name 'x'.
+
+        var e = (x: number, y: string): void;
+                  ~
+!!! ')' expected.
+                                     ~
+!!! ',' expected.
+                                      ~
+!!! Variable declaration expected.
+                                        ~~~~
+!!! Variable declaration expected.
+                                            ~
+!!! Expression expected.
+                 ~
+!!! Cannot find name 'x'.
+    }
+
+    module okay {
+        var a = () => { };
+
+        var b = (): void => { }
+
+        var c = (x) => { };
+
+        var d = (x: number, y: string) => { };
+
+        var e = (x: number, y: string): void => { };
+    }

tests/baselines/reference/emptyMemberAccess.errors.txt

@@ -1,11 +1,9 @@
-==== tests/cases/compiler/emptyMemberAccess.ts (2 errors) ====
+==== tests/cases/compiler/emptyMemberAccess.ts (1 errors) ====
     function getObj() {
 
        ().toString();
-         ~
-!!! '=>' expected.
-          ~~~~~~~~
-!!! Cannot find name 'toString'.
+        ~
+!!! Expression expected.
 
     }
 

tests/baselines/reference/es6ClassTest9.errors.txt

@@ -2,8 +2,8 @@
     declare class foo();
                      ~
 !!! '{' expected.
-                       ~
-!!! '=>' expected.
+                      ~
+!!! Expression expected.
     function foo() {}
              ~~~
 !!! Duplicate identifier 'foo'.

tests/baselines/reference/parser566700.errors.txt

@@ -1,4 +1,4 @@
 ==== tests/cases/conformance/parser/ecmascript5/RegressionTests/parser566700.ts (1 errors) ====
     var v = ()({});
-              ~
-!!! '=>' expected.
+             ~
+!!! Expression expected.

tests/baselines/reference/parserEmptyParenthesizedExpression1.errors.txt

@@ -1,8 +1,6 @@
-==== tests/cases/conformance/parser/ecmascript5/ErrorRecovery/parserEmptyParenthesizedExpression1.ts (2 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/ErrorRecovery/parserEmptyParenthesizedExpression1.ts (1 errors) ====
     function getObj() {
        ().toString();
-         ~
-!!! '=>' expected.
-          ~~~~~~~~
-!!! Cannot find name 'toString'.
+        ~
+!!! Expression expected.
     }

tests/baselines/reference/parserErrorRecovery_Expression1.errors.txt

@@ -1,4 +1,4 @@
 ==== tests/cases/conformance/parser/ecmascript5/ErrorRecovery/Expressions/parserErrorRecovery_Expression1.ts (1 errors) ====
     var v = ()({});
-              ~
-!!! '=>' expected.
+             ~
+!!! Expression expected.

tests/baselines/reference/uncaughtCompilerError2.errors.txt

@@ -1,9 +1,7 @@
-==== tests/cases/compiler/uncaughtCompilerError2.ts (2 errors) ====
+==== tests/cases/compiler/uncaughtCompilerError2.ts (1 errors) ====
     function getObj() {
        ().toString();
-         ~
-!!! '=>' expected.
-          ~~~~~~~~
-!!! Cannot find name 'toString'.
+        ~
+!!! Expression expected.
     }
 

tests/cases/compiler/arrowFunctionsMissingTokens.ts

@@ -0,0 +1,66 @@
+
+module missingArrowsWithCurly {
+    var a = () { };
+
+    var b = (): void { }
+
+    var c = (x) { };
+
+    var d = (x: number, y: string) { };
+
+    var e = (x: number, y: string): void { };
+}
+
+module missingCurliesWithArrow {
+    module withStatement {
+        var a = () => var k = 10;};
+
+        var b = (): void => var k = 10;}
+
+        var c = (x) => var k = 10;};
+
+        var d = (x: number, y: string) => var k = 10;};
+
+        var e = (x: number, y: string): void => var k = 10;};
+
+        var f = () => var k = 10;}
+    }
+
+    module withoutStatement {
+        var a = () => };
+
+        var b = (): void => }
+
+        var c = (x) => };
+
+        var d = (x: number, y: string) => };
+
+        var e = (x: number, y: string): void => };
+
+        var f = () => }
+    }
+}
+
+module ceci_nEst_pas_une_arrow_function {
+    var a = ();
+
+    var b = (): void;
+
+    var c = (x);
+
+    var d = (x: number, y: string);
+
+    var e = (x: number, y: string): void;
+}
+
+module okay {
+    var a = () => { };
+
+    var b = (): void => { }
+
+    var c = (x) => { };
+
+    var d = (x: number, y: string) => { };
+
+    var e = (x: number, y: string): void => { };
+}