From: dblack@...
Date: 2007-03-02T11:22:14+09:00
Subject: Re: Duck Typing Hash-Like Objects

Hi --

On Fri, 2 Mar 2007, Jacob Fugal wrote:

> On 3/1/07, Gary Wright <gwtmp01@mac.com> wrote:
>> My question is:  What is the least constraining test to determine
>> if you've got a hash-like object?  Is arg.respond_to?(:has_key?)
>> reasonable?  At first I thought a test for :[] would be great but
>> that catches strings also.  I'm thinking that if someone hands my
>> method a Hash or a HashWithIndifferentAccess or an OrderedHash or
>> a tree of some sort, I'd like to be able to accept all of them.
>
> I'd do it like this:
>
> def foo(duck)
>   # if the duck claims to have keys and indexing, we'll just use it as 
> is
>   unless duck.respond_to?(:keys) and duck.respond_to?(:[])
>     # otherwise, we'll ask it to turn itself into a hash for us
>     if duck.responds_to?(:to_hash)
>       duck = duck.to_hash
>     else
>       # not close enough to a hash...
>       raise ArgumentError, "want something with keys and indexing,
> or that supports to_hash"
>     end
>   end
>   ...
> end

Or you could just do:

   duck[whatever]....

and rescue the exception(s), possibly cascading down into a to_hash
operation.  You might as well fail without bothering with the
respond_to? calls -- just ask the object to do what it's supposed to,
and handle the error cases.


David

-- 
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