This last form could be chosen as soon as the value assigned is "big".

Desugaring of ?: operator

In general, it seems like desugared code should never be shown in error messages. Only show code fragments when necessary, and only show code that actually appears in the source files.

Code:

    class A { ?Integer n; }

    void main(String [] args) {
        let a2 = new A(n: new Integer(2));
        Integer n2 = a2.n != null ? a2.n : new Integer(1);
    }

Old message:

    The value if(`!=`(a2.n(), null))
                 a2.n()
              else
                 new Integer(1)
    cannot be assigned to n2 because it might be null.

New message:

    The value a2.n != null ? a2.n : new Integer(1)
    cannot be assigned to n2 because it might be null.

    Attempt to assign a possibly-null value to n2

I just fixed the problem of these extreme long error messages. The parser has some difficulties with generating error messages in case of a lookahead.

But I haven't a good idea yet how to improve the normal parse exception messages.

-- ArjanB

I just tested the compiler on the real-world code that inspired this example to see how long it would actually take to report the error - I gave up after 20 minutes. :-)

Here's a particularly awful example of this problem:

void main(String[] args) 
{
  String[] foo = [];
  args.foreach(String s => {
    if (true) {
      foo = null;
    } else {
      foo = foo.concat(["bar"]) //Note missing semicolon.
    }
  });  
}

This takes long enough to report a message that you're likely to think the compiler's gone into a loop, especially if it's in a real source file not just this tiny example. Finally, it produces more than 700 lines of parse alternatives, most of which look something like these:

    <IDENT> "=>" "{" "if" "(" <IDENT> ")" "{" <IDENT> "=" "null" ";" ";" ...
    <IDENT> "=>" "{" "if" "(" <IDENT> ")" "{" <IDENT> "=" "null" ";" "++" ...
    <IDENT> "=>" "{" "if" "(" <IDENT> ")" "{" <IDENT> "=" "null" ";" "--" ...
    <IDENT> "=>" "{" "if" "(" <IDENT> ")" "{" <IDENT> "=" "null" ";" "null" ...

Useless parsing exceptions

Sometime if you forget a ";" or a ")" or something else then you get a very long parse exception messages. It's not helpfull to get 20 lines of possible alternatives of wich half are multi-token alternatives.

No good example at the moment.

Missing arguments in constructor call - no arguments provided

Code:

class A {
 String B;
 String C = "test";
 ?String D;
}

void main(String[] args) {new A();}

Old message:

Class test.A has the following fields:
java.lang.String B, java.lang.String C = "Test", ?java.lang.String D
Please provide values for the fields, at least for those with no default value.
The syntax is:
new test.A(field1: value1, ..., fieldN: valueN) 

New message:

Fields of class test.A require initial values.
Use the following syntax:
 new test.A(B: value, D: value)

Class test.Foo has the following fields:
 java.lang.String B
 java.lang.String C = "test"
 ?java.lang.String D

BrynKeller - 30 Apr 2003

Missing arguments in constructor call - at least one argument provided

Code:

class A {
 String B;
 String C = "test";
 ?String D;
}

void main(String[] args) {new A(B: "foo");}

Old message:

Class test.A has the following fields:
java.lang.String B, java.lang.String C = "Test", ?java.lang.String D
Please provide values for the fields, at least for those with no default value.
The syntax is:
new test.A(field1: value1, ..., fieldN: valueN) 

New message:

Fields of class test.A require initial values.
These fields are missing:
 ?java.lang.String D

BrynKeller - 30 Apr 2003

To do

To do

Unsatisfiable constraints

Unsatisfiable constraints

Too long message when calling a method with a wrong argument.

Too long message when calling a method with a wrong argument.

Recent improvements

Recent improvements

Null values in assignment

Null values in assignment

Code:

Code:

Old message:

Old message:

New message:

New message:

Null values in synchronization

Null values in synchronization

Code:

Code:

Old message:

Old message:

New message:

New message:

Code:

Code:

Old message:

Old message:

New message:

New message:

TOC: No TOC in "Dev.BadErrorMessages"

To do

To do

Unsatisfiable constraints

Too long message when calling a method with a wrong argument.

?List<String> list = null;
list.get(1);

No possible call for get.
Arguments: (?java.util.List<java.lang.String>, nice.lang.byte)
Possibilities:
nice.lang.int get(java.util.Calendar, nice.lang.int)
java.lang.Object get(sun.reflect.FieldAccessor, ?java.lang.Object)
<K, V> V get(java.util.Dictionary<K, V>, ?java.lang.Object)
<E> E get(java.util.List<E>, nice.lang.int)
nice.lang.char get(String s, int index)
nice.lang.double get(nice.lang.Array<double>, int)
nice.lang.float get(nice.lang.Array<float>, int)
nice.lang.char get(nice.lang.Array<char>, int)
nice.lang.long get(nice.lang.Array<long>, int)
nice.lang.int get(nice.lang.Array<int>, int)
nice.lang.short get(nice.lang.Array<short>, int)
nice.lang.byte get(nice.lang.Array<byte>, int)
nice.lang.boolean get(nice.lang.Array<boolean>, int)
<Any T> T get(nice.lang.Array<T>, int)
nice.lang.boolean get(int x, int bit)
nice.lang.boolean get(long x, int bit)
<Any K, Any K0, Any V0, Any V | K <: K0, V <: ?V0> ?V0 get(java.util.Map<K, V>, K0)
<Any T, Any U> U get(Field, T)

Recent improvements

Recent improvements

Null values in assignment

Null values in assignment

Null values in synchronization

Null values in synchronization

To do

Recent improvements

We can keep track of recent improvements. This way, there can be a debate about the change, if somebody doesn't like it, or thinks it could be further improved.

Each entry should follow this format: code, old message, new message. To add an entry, you can copy paste the template at the bottom, and fill it in.

Null values in assignment

Code:

  List<String> l = null;

Old message:

null might be null

New message:

The value null cannot be assigned to l because it might be null.

To allow l to contain the null value, it should be declared as:
?java.util.List<java.lang.String> l

-- DanielBonniot - 30 Apr 2003

Null values in synchronization

Code:

  ?String s0 = null;
  synchronized (s0) {};

Old message:

s0 might be null

New message:

Synchonization must be done on a non-null object.
s0 might be null.

-- DanielBonniot - 30 Apr 2003

template

Code:

Old message:

New message:

%META:TOPICINFO{author="ArjanB" date="1051570138" format="1.0" version="1.1"}% %META:TOPICPARENT{name="NiceCompiler"}% An overview of errormessages that needs improvement.

class A{}
class B extends A{}

interface I<A T>{}
class X<T | A <: T> implements I<T>{}

void foo(I<B>);

foo(@X){}

Type error in method body "foo":
mlsub.typing.TypingEx: Not satisfiable [NO MESSAGE]

-- ArjanB