在Lua中调用作为参数传递的函数。

我有以下代码

Option = { }

function Option.nothing( )
  local self = { isNone = true, isSome = false }

  function self:orElse( alt )
    return alt
  end

  function self:map( f )
    return Option.nothing( )
  end

  function self:exec( f )
  end

  function self:maybe( alt, f )
    return alt
  end

  return self
end

function Option.just( val )
  local self = { isNone = false, isSome = true }
  local value = val

  function self:orElse( alt )
    return value
  end

  function self:map( f )
    return Option.just( f(value) )
  end

  function self:exec( f )
    f( value )
  end

  function self:maybe( alt, f )
    return f(value)
  end

  return self
end

function printOpt( opt )
  local str = opt.maybe( "Nothing", function(s) return "Just " .. s end )
  print( str )
end

x = Option.nothing( )
y = Option.just( 4 )

printOpt(x)
printOpt(y)

但我在这里一直得到“尝试调用本地'f'（nil值）”：

  function self:maybe( alt, f )
    return f(value)
  end

似乎我在调用作为参数传递的函数时遇到了麻烦。