Cannot unify recursive union

[lpil]

I attempted to define the iolist type like so:

type iolist
  = list iolist
  | string

val my_iolist : iolist
let my_iolist =
  ["h", ["i", ["!"]]]

This does not compile

louis ~/projects/learning-alpaca/hello_world $ rebar3 eunit
===> Verifying dependencies...
===> Compiling hello_world
===> Alpaca 0.2.8: compiling hello_world.alp

===> Alpaca compile error:
(╯°□°）╯︵ ┻━┻ {cannot_unify,hello_world,0,t_string,error}
Sorry, we do not have a proper message for this error yet.

Is this a bug or by design?

Thanks,
Louis

[j14159]

Definite bug, thanks for finding this! The error atom suggests that the typer is assuming success somewhere where it should be checking for an error tuple, e.g. {error, _}. This can happen because typ_of/2 usually returns {Type, NextTypeVariableNumber :: integer()} or {error, term()}.

[j14159]

I think I know what this is now, roughly: alpaca_typer.find_covering_types/4 (or rather try_types/6) isn't looking at list iolist as a possible alias for list string. Not 100% sure this is it but looking likely.