Clearer lexer errors for common cases.

This adds a clearer error specifically for the common case of using single quotes for string literals. It also makes minor clarity improvement to existing errors.

Fixes graphql/graphiql#198

Author: leebyron

src/language/__tests__/lexer-test.js

@@ -23,7 +23,8 @@ describe('Lexer', () => {
 
     expect(() => lexOne('\u0007')
     ).to.throw(
-      'Syntax Error GraphQL (1:1) Invalid character "\\u0007"'
+      'Syntax Error GraphQL (1:1) ' +
+      'Cannot contain the invalid character "\\u0007"'
     );
 
   });
@@ -106,7 +107,8 @@ describe('Lexer', () => {
 
 `)
     ).to.throw(
-      'Syntax Error GraphQL (3:5) Unexpected character "?".\n' +
+      'Syntax Error GraphQL (3:5) ' +
+      'Cannot parse the unexpected character "?".\n' +
       '\n' +
       '2: \n' +
       '3:     ?\n' +
@@ -178,11 +180,18 @@ describe('Lexer', () => {
 
     expect(
       () => lexOne('"')
-    ).to.throw('Syntax Error GraphQL (1:2) Unterminated string');
+    ).to.throw('Syntax Error GraphQL (1:2) Unterminated string.');
 
     expect(
       () => lexOne('"no end quote')
-    ).to.throw('Syntax Error GraphQL (1:14) Unterminated string');
+    ).to.throw('Syntax Error GraphQL (1:14) Unterminated string.');
+
+    expect(
+      () => lexOne('\'single quotes\'')
+    ).to.throw(
+      'Syntax Error GraphQL (1:1) Unexpected single quote character (\'), ' +
+      'did you mean to use a double quote (")?'
+    );
 
     expect(
       () => lexOne('"contains unescaped \u0007 control char"')
@@ -406,7 +415,9 @@ describe('Lexer', () => {
 
     expect(
       () => lexOne('+1')
-    ).to.throw('Syntax Error GraphQL (1:1) Unexpected character "+"');
+    ).to.throw(
+      'Syntax Error GraphQL (1:1) Cannot parse the unexpected character "+".'
+    );
 
     expect(
       () => lexOne('1.')
@@ -417,7 +428,9 @@ describe('Lexer', () => {
 
     expect(
       () => lexOne('.123')
-    ).to.throw('Syntax Error GraphQL (1:1) Unexpected character "."');
+    ).to.throw(
+      'Syntax Error GraphQL (1:1) Cannot parse the unexpected character ".".'
+    );
 
     expect(
       () => lexOne('1.A')
@@ -572,19 +585,29 @@ describe('Lexer', () => {
 
     expect(
       () => lexOne('..')
-    ).to.throw('Syntax Error GraphQL (1:1) Unexpected character "."');
+    ).to.throw(
+      'Syntax Error GraphQL (1:1) Cannot parse the unexpected character ".".'
+    );
 
     expect(
       () => lexOne('?')
-    ).to.throw('Syntax Error GraphQL (1:1) Unexpected character "?"');
+    ).to.throw(
+      'Syntax Error GraphQL (1:1) Cannot parse the unexpected character "?".'
+    );
 
     expect(
       () => lexOne('\u203B')
-    ).to.throw('Syntax Error GraphQL (1:1) Unexpected character "\\u203B"');
+    ).to.throw(
+      'Syntax Error GraphQL (1:1) ' +
+      'Cannot parse the unexpected character "\\u203B".'
+    );
 
     expect(
       () => lexOne('\u200b')
-    ).to.throw('Syntax Error GraphQL (1:1) Unexpected character "\\u200B"');
+    ).to.throw(
+      'Syntax Error GraphQL (1:1) ' +
+      'Cannot parse the unexpected character "\\u200B".'
+    );
   });
 
   it('lex reports useful information for dashes in names', () => {

src/language/lexer.js

@@ -211,7 +211,7 @@ function readToken(lexer: Lexer<*>, prev: Token): Token {
     throw syntaxError(
       source,
       position,
-      `Invalid character ${printCharCode(code)}.`
+      `Cannot contain the invalid character ${printCharCode(code)}.`
     );
   }
 
@@ -276,10 +276,22 @@ function readToken(lexer: Lexer<*>, prev: Token): Token {
   throw syntaxError(
     source,
     position,
-    `Unexpected character ${printCharCode(code)}.`
+    unexpectedCharacterMessage(code)
   );
 }
 
+/**
+ * Report a message that an unexpected character was encountered.
+ */
+function unexpectedCharacterMessage(code) {
+  if (code === 39) { // '
+    return 'Unexpected single quote character (\'), did you mean to use ' +
+      'a double quote (")?';
+  }
+
+  return 'Cannot parse the unexpected character ' + printCharCode(code) + '.';
+}
+
 /**
  * Reads from body starting at startPosition until it finds a non-whitespace
  * or commented character, then returns the position of that character for