"pop_frame: Empty call stack" when folding a tuple of maps

[spbots]

See the following example for the issue and a workaround:

import io, koto, string.print

export main = ||
  map_is_valid = |m| m.get("bar") != ()
  tuple_of_maps = ({foo: 2}, {}, {foo: 4, bar: 2})

  # uncomment the following print for the following error:
  #
  #     pop_frame: Empty call stack
  #      --> fold_test.koto - 1:1
  #        |
  #      1 | import io, koto, string.print
  #        | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  #
  # print "is_valid 2: {}" (map_is_valid tuple_of_maps[2])

  print "is_valid 2: {}" (tuple_of_maps[2].get("bar") != ())

  n_valid = tuple_of_maps.fold(0, |acc, m| acc + (if map_is_valid(m) then 1 else 0))

  print "n_valid: {}" n_valid

[spbots]

I've played around with the minimal case a couple times — it looks like the issue is not actually caused by trying to fold the tuple_of_maps but by calling the map_is_valid function before the fold.

[irh]

Thanks for the report 👍

[spbots]

Thanks for the fun language! It looks like it's not necessary to use the map_is_valid function for the fold e.g.

import io, koto, string.print

export main = ||
  map_is_valid = |m| m.get("bar") != ()

  tuple_of_maps = ({foo: 2}, {}, {foo: 4, bar: 2})

  # okay:
  # x = (tuple_of_maps[2].get("bar") != ())

  # explosion:
  # x = (map_is_valid tuple_of_maps[2])

  n_valid = tuple_of_maps.fold(
    0,
    |acc, mm| debug acc + (if mm.get("bar") != () then 1 else 0))

[irh]

It turns out that calling any function before calling fold() can trigger the issue, it seems to be an unfortunate fluke that this hasn't come up before 🙃

f = || 1, 2, 3
f().fold 0 |x, n| x += n # error: empty call stack