Weird parser error

[peter-ahe-google]

This program:

final int IDENTIFIER_TOKEN = 0;

class Token {
  int kind;
}

class Fisk {
  Token peekAfterType(Token) {
    assert(token.kind === IDENTIFIER_TOKEN);
    Token peek = token.next;
  }
}

main() {
  new Fisk().peekAfterType(null);
}

Gives this error:

'foo.dart': Error: line 10 pos 11: semicolon expected
    Token peek = token.next;
          ^

Compare this to how dartc handles this problem:

foo.dart/foo.dart:10: type "Token" expected, but "PARAMETER" found
     9: assert(token.kind === IDENTIFIER_TOKEN);
    10: Token peek = token.next;
foo.dart/foo.dart:10: no such type "Token"
     9: assert(token.kind === IDENTIFIER_TOKEN);
    10: Token peek = token.next;
foo.dart/foo.dart:9: cannot resolve token
     8: Token peekAfterType(Token) {
     9: assert(token.kind === IDENTIFIER_TOKEN);
foo.dart/foo.dart:10: cannot resolve token
     9: assert(token.kind === IDENTIFIER_TOKEN);
    10: Token peek = token.next;

These are all warnings (as they should be) and the program runs and throws an dynamic error:

/var/folders/++/++3l3+++6+0++4RjPqRgNE+-QFE/-Tmp-/dart_foo.dart1176370598895860049.js:17663: TypeError: Object #<unnamedc08e84$Fisk$Dart> has no method 'token$getter'
  var peek = this.token$getter().next$getter();
                  ^
TypeError: Object #<unnamedc08e84$Fisk$Dart> has no method 'token$getter'

And here is what frogsh reports:

foo.dart:9:12: warning: can not resolve "token" on "Fisk"
    assert(token.kind === IDENTIFIER_TOKEN);
           ^^^^^
foo.dart:9:12: warning: token is not defined anywhere in the world.
    assert(token.kind === IDENTIFIER_TOKEN);
           ^^^^^
foo.dart:10:18: warning: can not resolve "token" on "Fisk"
    Token peek = token.next;
                 ^^^^^
foo.dart:10:18: warning: token is not defined anywhere in the world.
    Token peek = token.next;
                 ^^^^^

node.js:201
        throw e; // process.nextTick error, or 'error' event on first tick
              ^
NoSuchMethodException - receiver: '[object Object]' function name: 'get:token' arguments: []]

[DartBot]

This comment was originally written by @mhausner

---

The error message from the VM may seem weird, but I think it is correct.

method peekAfterType has one untyped parameter named Token, which hides the class name Token. The line:

 Token peek = token.next;

therefore starts with the expression Token, after which the compiler expects a semicolon.

Are you saying this interpretation of the semantics of your code snipped it incorrect?

[peter-ahe-google]

The error message from the VM is not correct. The program contains a lot of warnings, but the VM normally doesn't produce these warnings. So instead the VM should compile the code and produce a dynamic error on this line:

Token peek = token.next;

This is because "token" is unresolved, so it turns into "this.token". So the dynamic error would be:

NoSuchMethodException - receiver: 'Instance of 'Fisk'' function name: 'get:token' arguments: []]

Similar to how it would treat this program:

class Fisk {
  test() {
    token;
  }
}

main() {
  new Fisk().test();
}

Unhandled exception:
NoSuchMethodException - receiver: 'Instance of 'Fisk'' function name: 'get:token' arguments: []]
 0. Function: 'Object.noSuchMethod' url: 'bootstrap' line:93 col:3
 1. Function: 'Fisk.test' url: '/Users/ahe/Dart/all/dart.googlecode.com/dart/foo.dart' line:3 col:10
 2. Function: '::.main' url: '/Users/ahe/Dart/all/dart.googlecode.com/dart/foo.dart' line:8 col:18

VM has no concept of warnings, none are produced (all according to spec, AFAIK).
Using a variable incorrectly should be a compile time error, as in this:

foo(x) {
  x y z;
}

[DartBot]

This comment was originally written by @mhausner

---

The issue in the code is not that token is undefined. The language spec clearly defines what happens when an identifier cannot be resolved.

The issue is that what looks like a declaration:

Token peek = ...

is not a valid declaration. Token is not a type, it's the name of the formal parameter. Thus, this line is a malformed expression and the error message is correct.

I agree that the error message is misleading, but that is because you expect the line to be something completely different from what it actually is.

[peter-ahe-google]

Where in the spec does it say that

Token peek = ...

is not a valid declaration?

The grammar says that it is a variable declaration statement. The grammar is context free.

The specification says the following about type annotations:

"Static type annotations are used in variable declarations (5) (including formal parameters (6.2)) and in the return types of functions (6). Static type annotations are used during static checking and when running programs in checked mode. They have no effect whatsoever in production mode."

[gbracha]

Token peek == token.next;

is a variable declaration. The fact that Token is the name of a formal parameter does not change that (it's a context-free grammar).

[iposva-google]

In the latest VM builds we do not complain about the type confusion any longer. Although we are not resolving the access to token properly: Issue #1805

---

Set owner to @mhausner.
Added Accepted label.
Marked this as being blocked by #1805.

[DartBot]

This comment was originally written by @mhausner

---

This is now fixed. The VM correctly reports a runtime error when it can not resolve the call to getter for 'token':

NoSuchMethodException : method not found: 'get:token'
Receiver: Instance of 'Fisk'
Arguments: []
 0. Function: 'Object.noSuchMethod' url: 'bootstrap' line:717 col:3
 1. Function: 'Fisk.peekAfterType' url: 'file:///Users/hausner/tmp/t.dart' line:10 col:24
 2. Function: '::main' url: 'file:///Users/hausner/tmp/t.dart' line:15 col:27

In checked mode, we get an error, as expected:

$ dart --enable_type_checks=true ~/tmp/t.dart
'file:///Users/hausner/tmp/t.dart': Error: line 10 pos 5: using 'Token' in this context is invalid
    Token peek = token.next;
    ^

---

Added Fixed label.

[peter-ahe-google]

Very nice. Thank you!