Custom error message for missing "end"

[kostmo]

I've found a common mistake in my own swarm-lang code to be a missing end keyword at end of a def. One reason for the frequency of this error is probably that the languages I, and I suspect most others, are most familiar with (e.g. Python and Haskell) do not require this keyword at the end of a function.

The error manifests itself far away (at the end of the file) and as the message "unexpected end of input".
Since it is a common error, could we special-case this error to have a more user-friendly message? Perhaps in the parser we can count to make sure that the number of end keywords matches the number of def keywords.

[byorgey]

Yes, that sounds like it should be feasible. If we work a little harder we can probably even pinpoint the place where the missing end likely occurred --- it will be the first place we try to parse a def inside the body of another def. Currently that is syntactically valid but would fail if we try to typecheck it. Perhaps we could even refactor the parser a bit to make it a syntax error for a def to show up inside the body of another def. i.e. we can add a flag to some parsers indicating whether we are at the "top level" or not, and allow def only when at the top level. Then if we saw a def at the wrong level we could say "Unexpected def. Perhaps you have a missing end?" or something like that.

[ussgarci]

I'd like to work on this issue. 😅

[xsebek]

Just to update, since #1087 the def inside def is valid:

def a =
  def b = 1 end;
  return (b + b)
end;
a

It's still a good bet that the end should be before the next def though. 🙂