eff.lua

ONE-SHOT Algebraic Effects for Lua!

installation

$ luarocks --local install eff

usage

eff provides four objects, inst, perform and handler.

effect instantiation and invocation

inst generates an effect instance. perform invoke the passed effect.

local Write = inst() -- instantiation
perform(Write("Hello!")) -- invocation

effect handler

handler(h)

handler requires a table and returns a handling function. The table has fields, a value handler as val and [Eff] as an effect handler corresponding to Eff. "Handling an expression with a handler" is translated into an application passing a thunk, containing the expression, to the handling function.

local printh = handler {
  val = function(v) print("printh ended", v) end,
  [Write] = function(k, arg)
      print(arg)
      k()
    end,
  [Read] = function(k)
      return k("baa")
    end
}

printh(function()
  local x = perform(Write("hello"))
  return x
end)

--[[ prints:
hello
printh ended    nil
]]

restriction about continuation

The continuation effect handler received is ONE-SHOT, in other words, the continuatoin cannot run more than twice.

handler({
  val = function(v) print("printh ended", v) end,
  [Write] = function(k, arg)
      print(arg)
      k()
      k() -- call continuation twice
    end
})(function()
  perform(Write("Foo"))
end)

--[[prints
lua: ./eff.lua:91: ./eff.lua:82: continuation cannot be performed twice
stack traceback:
        [C]: in function 'error'
        ./eff.lua:91: in local 'printh'
        ../example/example.lua:28: in main chunk
        [C]: in ?
]]

LICENSE

MIT