Re: PiL 3 and pcall

["Pierre Chapuis" <catwell@...>]

> On 31/01/2013, at 9:46 PM, "Pierre Chapuis" <catwell@archlinux.us> wrote:
>
>> In PiL 3 chapter 8 there is this exercise:
>>
>>    "Can you find any value for f such that the call pcall(pcall,f)
>> returns false as its first result?"
>
>
> Does the absence of a value count?
>
> $ lua
> Lua 5.1.5  Copyright (C) 1994-2012 Lua.org, PUC-Rio
>> =pcall(pcall, (function() end)())
> false	bad argument #1 to '?' (value expected)
>
> pcall(pcall) also works, but then it's more obvious that I'm cheating and
> that f is not a value.

Nice trick, although I understood the exercise as: "write pcall(pcall,f)
in your code and it must return false as the first result". Another thing
I would not accept is modifying anything (e.g. pcall...) before calling
this.

fyi what I did is:

    local f = function()
      local second = false
      local h = function()
        if c then debug.sethook(nil,"r") end
        c = not c
        error()
      end
      debug.sethook(h,"r")
    end

    pcall(pcall,f)

It returns "false, nil"...

Roberto, will solutions to the exercises be published someday? ;)

-- 
Pierre Chapuis