Introducing a bottom type

[pascallouis]

Have we considered adding a bottom type to the type hierarchy? The following code

function (o) {
  switch (typeof o) {
    case 'object': only object processing
    case 'string': only string processing
    ...
  }
}

could be annotated

function (o : nothing) {
  ...
}

The name nothing comes from Scala (it used to be called All but this was found confusing).

Adding a bottom type would allow the meet of two types to be a total function. This is usefull to type check

function (a : (Date,String), b : (Date,Number)) : Date {
  if (a == b) {
    return a;
  } else {
    ...
  }
}

which is correct since the guard a == b is true only if a's type is Date and b's type is Date (the meet of a's type and b's type).

[lth]

Cormac, Dave -- can we have some comments on this no later than the next f2f (sooner is better).

Pascal, the proposal period is really over. Do we need this change to be type sound? What are the practical benefits?

[lth]

We have a bottom type: the empty union. Pascal and Cormac will chat about how / if this fits into the type system.

[lth]

Cormac, Pascal --

we need some movement on this one. What are your thoughts?

[pascallouis]

Based on the rules http://wiki.ecmascript.org/doku.php?id=clarification:type_system#union_types the empty union type () is a natural bottom type.