example.lua

local TypeCase = require 'TypeCase'.TypeCase
local SomeClass = require 'SomeClass'.SomeClass

local function isPositve(x)
  return type(x) == 'number' and x > 0
end

local result = TypeCase(2) {
  'string', function ()
    return 'string two'
  end,
  isPositive, function ()
    return 'positive number two'
  end,
  'number', function ()
    return 'number two'
  end,
  SomeClass, function ()
    return 'class two'
  end,
  'nil', function ()
    return 'nil two'
  end,
  nil, function ()
    return 'something else two'
  end,
}

print(result) -- prints 'positive number two'

TypeCase.lua

--- Control flow functions

local function TypeCase(obj, fallthroughHandler)
  return function (args)
    for i = 1, #args, 2 do
      local signature, action = args[i], args[i + 1]
      local function doAction(o)
        if type(action) == 'function' then
          return action(o)
        else
          return action
        end
      end
      if signature == nil then
        return doAction(obj)
      elseif type(signature) == 'string' then
        if type(obj) == signature then
          return doAction(obj)
        end
      elseif type(signature) == 'table' then
        local mt = getmetatable(obj)
        if mt and mt.__index == obj then
          return doAction(obj)
        end
      elseif type(signature) == 'function' then
        if signature(obj) then
          return doAction(obj)
        end
      else
        error('invalid TypeCase signature: ' .. tostring(signature))
      end
    end

    if fallthroughHandler then
      fallthroughHandler(obj)
    end
  end
end

local function ETypeCase (obj)
  return TypeCase(obj, function ()
    error('An object fell through ETypeCase expression: ' .. tostring(obj))
  end)
end

return {
  TypeCase = TypeCase,
  ETypeCase = ETypeCase,
}