close.lua

local eff = require('eff')
local inst, perform, handlers = eff.inst, eff.perform, eff.handlers

local Defer = inst()
local runDefer = function(th)
  local tasks = {}
  return handlers{
    function(x)
      for i = #tasks, 1, -1 do
        tasks[i]()
      end

      return x
    end,
    [Defer] = function(k, task)
      table.insert(tasks, task)
      return k()
    end
  }(th)
end

local Close = inst()
local closer = function(resr, clos)
  local close = function()
    clos(resr)
  end

  return setmetatable({ resource = resr, close = close }, {__index = resr})
end
local close = function(resr, clos)
  return perform(Close(closer(resr, clos)))
end

local runClose = function(th)
  local h = handlers{
    function(x) return x end,
    [Close] = function(k, res)
      perform(Defer(function() return res.close() end))
      return k(res.resource)
    end,
  }

  return runDefer(function()
    return h(th)
  end)
end

---

runClose(function()
  local fileBuf = close(io.open("src.txt"), io.close)
  local fileWriter = close(io.open("dst.txt", "a+"), io.close)

  for line in fileBuf:lines() do
    print(line)
    fileWriter:write("> " .. line .. "\n")
  end
end)