Lua when v2

Yes! Thanks to u/topchetoeuwastaken , I was able to rewrite it so you can now use nil in it. null is only used internally, so you don't have to worry about it.
It is inspired by Kotlin's when, though it currently only has a sub set of features (I might add the other features later).
Imagine you want to map a value to another value. If you don't need nil and if the mapping is straight forward, you could use a table. If it gets more complicated, you need a long ifelse chain like so:

```Lua
if x == a then
 return 1
elseif x == b then
 return 2
elseif x == c then
 return 3
else
 return 0
end
```

This has a couple issues:
- repeating return, because if is not an expression, so you can't write `return if ...`
- repeating comparison `x ==`
A `when` expression can make this more readable:

```
return when(x,
 a, 1,
 b, 2,
 c, 3,
 0   -- else case
)
```

I use this to map poker cards to illustrations. What's special here is that most cards of the same rank have the same illustration, but some have different ones, based on rank, which requires the nested `_when`:

--[[
returns the drawable to use as
the given card's illustration.
]]
function gfx.get_illu(c)
 local d = r.drawable
 local cr = card.rank
 local cs = card.suit
 local rank = card.get_rank(c)
 local suit = card.get_suit(c)
 return when(rank,

  -- gestalts --
  nil,
   _when(c,
    card.joker1, d.isa,
    card.joker2, d.ariane),

  -- artifacts --
  cr.ace,
   _when(suit,
    cs.spades, d.shield,
    cs.clubs,  d.knife,
    cs.hearts, d.king_in_yellow,
    cs.diamonds, d.plant),

  -- replica --
   2, d.klbr,
   3, d.eulr,
   4, when(suit,
       cs.clubs, d.adlr_knife,
       d.adlr),
   -- special case 5: lstr
   -- has four variants
   5, _when(suit,
       -- todo draw the other
       -- lstr variants
       cs.spades,d.lstr,--space
       cs.clubs, d.lstr_combat,
       cs.hearts,d.lstr_eaten,
       cs.diamonds, d.lstr),
   6, d.arar,
   7, d.star,
   8, d.stcr,
   9, d.flkr,
  10, d.mnhr,

  -- bosses --
  cr.jack,  d.corrupted_mnhr,
  cr.queen, d.chimera,
  cr.king,  d.corrupted_flkr)
end

Here is the implementation, it's currently written in PICO-8 Lua, so if you want to use it somewhere else, you'd have to replace the `add` with a `table.insert`:
null = {}
error_thunk = {}

--[[
pattern matches with optional
else fallback. if no else is
given and nothing matches, it
crashes.

When nesting whens, use "_when"
for all nested whens and only
"when" for the outer one.
]]
function when(s, ...)
 local result = _when(s,...)
 if(result == error_thunk) then
   -- no match (not exhaustive)
  print("when did not match " ..
    tostring(subject),0,0,8)
  print("result: " ..
   tostring(result))
  print("m:" ..
   tostring(matches))
  for i, v in ipairs(matches) do
    print(tostring(v[1]) ..
    " -> " .. tostring(v[2]))
  end
  local now = t()
  local crashtime = now + 4
--while(t() < crashtime )do end
  error("when not exhaustive!")
 elseif(result == null) then
  return nil
 end
 return result
end

--[[
Use this for nested whens.
]]
function _when(subject, ...)
 if subject == nil then
  subject = null
 end
 -- also replace nil in args
 local args = {}
 for i=1, select("#", ...) do
  local a = select(i, ...)
  if(a == nil) then
   add(args, null)
  else
   add(args, a)
  end
 end

 local matches =
  partition(args,2)

 local result = nil
 for _,cond in ipairs(matches)do
  if #cond == 1 and
   result == nil then
   result = cond[1]
  elseif subject == cond[1] and
   result == nil then
   result = cond[2]
  end
 end

 -- match found!
 if result != nil then
  return result
 end

 -- no match
 return error_thunk
end