From: James Edward Gray II <james@...>
Date: 2009-09-04T11:56:56+09:00
Subject: Re: Type checking function parameters

On Sep 3, 2009, at 7:30 PM, Eleanor McHugh wrote:

> On 3 Sep 2009, at 23:47, Nick Green wrote:
>> I see the idea, and it is tempting.  For code written for me to be  
>> used
>> by me it is in fact ideal.  However, I guess I have two follow up
>> questions, both about failing gracefully:
>>
>> What is the "right" way to make sure the errors these functions  
>> report
>> are nice to people who may use the functions/libraries I code?   
>> "Foo.bar
>> takes a Duck, not a Cow" would be a very helpful error message.   
>> object
>> of type Cow does not have method of type quack, is also not a bad  
>> error
>> message, but it would be nice to let the user know which type of  
>> object
>> in fact does have the method "quack" (presumably it would not  
>> always be
>> as obvious as "quack"==>"duck").  Or is the ruby way, since its an
>> interpereted language, for a user of my library to crack open the  
>> ruby
>> or rdoc/ri and look?
>
> The Ruby way isn't to try and defend against behaviour which is out  
> of your control. By publishing an API for your library you provide a  
> contract and it is the responsibility of the programmer using your  
> library to respect and conform to that contract. Whilst this can  
> seem quite daunting if you come from a language such as Java, you'll  
> find that it soon becomes a great liberator - not least because all  
> objects that respond to a particular message can be used  
> interchangeably.
>
> Likewise exception handling allows you to define clean error  
> recovery based on scope.
>
>> What is the "right" way to make sure that, if my libraries or  
>> functions
>> are misused by someone (possibly me :p), the user of the script/ 
>> program
>> receives a non-programmer-targeted friendly error message like  
>> "Whoops,
>> we sent a Cow to do a Ducks job.  Feel free to report the bug to
>> cows.and.ducks.sorting.co@notarealemail.com.  Make sure to include  
>> the
>> following: <normal error dump>"?
>
>
> In the general case there is no way to know the full set of objects  
> which can respond to a given message at a given time: a duck may  
> quack, but so for that matter might a duck hunter or a tape  
> recorder. This holds in Ruby because the runtime behaviour of  
> objects is mutable - methods might be added and removed at any time  
> during program execution. This is why it's much more useful to  
> rescue runtime exceptions and handle them elegantly than to worry  
> about the class of an object.
>
> So just relax and let duck typing happen, you'll find those tight  
> controls your brain's learned with other languages really are  
> irrelevant in Ruby.

That was so beautiful I almost teared up.  ;)

James Edward Gray II