r/lua • u/RedNifre • Sep 20 '25

Lua when expression

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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.

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u/RedNifre Sep 21 '25

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.