[Haskell-beginners] Further constraining types

[Christopher Howard]

On 08/06/2011 12:54 PM, Thomas wrote:
>
>> (==) (Natural a) (Natural b) | a == b = True
>> | otherwise = False
>
> Essentially the same, but I usually write this a little shorter as
> (==) (Natural a) (Natural b) = a == b
>
>> (Natural a) * (Natural b) | a < 0 || b < 0 = Indeterminate
>> | otherwise = Natural ((Prelude.*) a b)
>
> Why this? A 'Natural x' should already guarantee that 'x' is not
> negative (otherwise it would be 'Indeterminate').
>
>> Thoughts?
>
> It seems very reasonable to me.
>
> Regards,
> Thomas
>
>
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One more quick question: If I hide my actual constructors in favor of a 
smart constructor, is it no longer possible for me to do direct pattern 
matching on the values? E.g., the compiler does not allow this:

analyze (Natural 5) = "It's a five!!!"

...so I have to do this:

analyze a | natural 5 = "It's a five!!!"

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