Lua when

-- less difficult than nil
null = {}
-- see haskell
error_thunk = {}

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

Don't pass nil, except for the
subject. Use null instead
everywhere else, or it doesn't
work correctly.

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(s, ...)
 local subject = s
 if subject == nil then
  subject = null
 end

 local matches =
  partition({...},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