better errors in parser

[johnynek]

The parser needs better error messages. When we fail to parse e.g. missing ) it does not give very good messages.

One interesting law: if we make a random package and then modify a random character in it, we should either continue to parse (it was a legal change) or we should point to the position where we made the change.

This is probably hard to pass that law, but it would be awesome if we could.

[johnynek]

I'm pretty sure the built-in cons-list structure of Statement/Declaration is a disaster for parsing.

If you have:

x = 1
y = 4
{'x' : 'x' : x }

it is the dict that doesn't parse, but since it doesn't parse, the lets don't parse, so the error you get is that x=1 can't parse. This is pretty terrible.

A minimal solution would to change the parser to be something like P[List[A] => List[A]] where we give a function that if you pass the tail, it can make the list, so basically we just do { t => h :: t } in that function. Then we make a P[List[Declaration => Declaration]], finally we foldRight on that pushing the final declaration/statement into it.

[johnynek]

Here's another bad example:

5|export [ BinNat(), toInt, , toNat, toBinNat, next, add_BinNat, times2 ]
  ^

Note, we should point to the , , part, not to the export.