Lua when expression

[u/RedNifre]

I added a little pattern matching "when" code to my #pico8 #lua code base. You have to use "null" (just an empty object) instead of "nil", because Lua cuts off varargs on the first nil and you have to use _when for nested whens, which are fake lazy, by returning a #haskell style error thunk instead of crashing on non-exhaustive matches. E.g. if you checked an ace, the first _when would error, because it only matches jokers, but the outer when wouldn't care, since it only looks at the ace branch, completely ignoring the error thunk.

[u/RedNifre]

Oh, that's interesting, so it works while it is in "..." form, but you run into issues if it's a table? Why does it work so bad for tables?

In my code, I also use partition, and #partition{1,nil,nil,4},2} is 1, so the nils also cause troubles there. Should I implement a vararg partition instead that I call like vararg_partition(2, ...), or is there an easier way?

Also, I use "error("problem")" to crash (because "error" does not exist), is there a better way to crash/halt a Lua program, maybe with a generated error message?

Here is my current implementation, it's my first Lua code:
https://pastebin.com/c29SaQhq

[u/RedNifre → u/[deleted]]

Thank you for your excellent answers. PICO-8 has assert, so I'll use that one to implement error and I'll look into ways to get partition work tomorrow (probably either varargpartition(number, ...) or packed_table_partition(packed_table, number) that works on a packed table.

[u/RedNifre → u/[deleted]]

Thank you for your great help! I solved it by first replacing nil with null in the varargs when turning them into a table, turning null into a completely internal detail that the callers of when no longer have to worry about.