gfindtype type

[Ralf Laemmel]

Typeable is a superclass of Data.
So you would indeed imply Typeable by requiring Data.
However, you don't _need_ Data but only Typeable.
You need "Data x" to find "Typeable y"; once you find
"y" you don't need to _traverse_ any further into "y".
To summarise, requiring Typeable is simply more precise
as to what's needed.

Ralf

Stefan Holdermans wrote:

> Jim,
>
>>> Why is
>>>
>>> gfindtype :: (Data x, Data y) => x -> Maybe y
>>>
>>> and not
>>>
>>> gfindtype :: (Data x, Typeable y) => x -> Maybe y
>>>
>>> ?
>>
>>
>> Because you're not always interested in a subterm of the same type as 
>> the parent node.
>
>
> Ouch, answered your question before I really read it, i.e., before a 
> first cup of coffee.
>
> Sorry.
>
> Regards,
>
> Stefan
>
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-- 
Ralf Laemmel
VU & CWI, Amsterdam, The Netherlands
http://www.cs.vu.nl/~ralf/