From: gabriele renzi <rff_rff@...>
Date: 2004-09-08T18:40:06+09:00
Subject: Re: [RCR] Unified type conversion framework

Sam McCall ha scritto:

> Cool! Occasionally I think about doing something like this, so I can 
> have some nice way to remember the custom conversions I use.

I think most of us did, this is why I'm telling this is not something new :)

> Maybe you could sneak some extra conversions in with it, like
> [[1,2],[3,4]].as Hash # => {1=>2,3=>4}

The system is open, I did thought of this when thinking how unintuitive 
Hash[anArray] is . This is another example of the need for a better 
approach, I guess there are much more ;)

> Related to this is type-checking when you do want it (duck typing is 
> good, but I'd give it up in some cases to have an easy, readable way of 
> checking that I'm not passing a hash instead of an array).
> I'd really like to be able to do:
> def method(foo,bar)
>     foo.is! Hash
>     bar.can_be! Array
>     #...
> end
> where they do something like
> def is! t
>     raise TypeError.new("Expected a #{t} but got a " +
>            "#{self.type}: #{self.inspect}") unless is_a? t
> end
> 
> def can_be! t
>     raise TypeError.new("Can't convert to #{t} from "+
>            "#{self.type}: #{self.inspect}") unless can_be? t
> end
> 
> And can_be? is the does-this-conversion-exist method.

#is! is broken, because it relies on is_a? that is not powerful enough 
to really express a type, but #as (or #to.. I mean, the proposed system) 
is powerful enough to expres any kind of type, including your #is! :)

The same is said for can_be? Sure, I could introduce a method in the 
ConvTable that checks if a conversion path exists, but it won't work 
for, say,
  'ciao'.as Integer
because even if a String -> Integer conversion path do exist, you still 
have to check the whole string. So you end up needing #as again.


> This might even be rdoc-able, if it was at the top of a method. It'd 
> certainly make the ad-hoc documentation I write today executable.

exacylt what I was thinking of. The IdeaSpace is limited, it seem ;)