From: Robert Dober <robert.dober@...>
Date: 2006-08-07T00:17:29+09:00
Subject: Re: How to react on nil or wrong object-type as parameter

------=_Part_119914_4725452.1154877444141
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline

On 8/6/06, Yochen Gutmann <yoche2001@dergutemann.com> wrote:
>
> Hi,
>
> I was just wondering: how do I react on a method-invocation with a
> nil-object instead of eg. String? How do I test it? And how can I assure
> that it is the right kind of object?
>
> class AClass
>   def a_method aString
>      puts aString+aString
>      puts aString.reverse
>    end
>
> end
>
> an_object =3D AClass.new
> an_object.a_method "hello"  # works
> an_object.a_method          3#-> undefined method
> an_object.a_method nil      #-> undefined method
>
>
> Does Duck-Typing mean "ah, the programmer always knows, which kind of
> objects this method expects. Thus never an error will occur."? This
> seems very optimistic to me if not naiv. You cant just trust that the
> any programmer who uses your class knows what exactly is happening in
> your method and which type of object is required.
>
> Thankful for any hint.


Well what a nice occasion. I have to reply fast because I will be
outnumbered fast ;)
I have for quite a while now  spoken up for some mechanisms for defensive
programming and early failure, without success so far.
What you can do e.g. is

def aMethod something
    raise WhateverException, "whatever info useful" unless String =3D=3D=3D
something

you might also distinguish sometimes between nil and other problems

However I have to conceede a point to the ducktypers (which is quite a nice
feature I never want to see disappear) *sometimes*
you want something to reply to #reverse and to #+, ( a String object might
not reply to these messages ).

So you could just fail or fail with a more meaningful message like this
   raise GreatExplanation, "sage sayings" unless something.respond_to?:reve=
rse

I still think it would be great to have an expressive syntax about this lik=
e
e.g;

def act actor : Actor
def act actor is Actor
def act actor where Actor =3D=3D=3D actor
def act actor : SomeThing

note that SomeThing could also be a module and ruby should check if the
instance methods of SomeThing are replied to by actor.
Thus just doing kind of an automated duck type check.

Hope you do not mind using your thread for some lobbying :)

Cheers
Robert

> Regards,
> Yochen
>
> --
> Posted via http://www.ruby-forum.com/.
>
>


--=20
Deux choses sont infinies : l'univers et la b=EAtise humaine ; en ce qui
concerne l'univers, je n'en ai pas acquis la certitude absolue.

- Albert Einstein

------=_Part_119914_4725452.1154877444141--