Re: [Haskell-cafe] Re: Haskell Zippers on Wikibooks: teasing! :)

[Dougal Stanton]

On Thu, Jul 16, 2009 at 12:11 PM, Heinrich
Apfelmus wrote:
>
>   Generic Programming: An introduction
>   http://www.cse.chalmers.se/~patrikj/poly/afp98/
>
> It's a bit verbose at times, but you only need the first few chapters to
> get an idea about polynomial functors (sums and pairs) and  mu .

Thanks, that's a really nice introduction, which seems to be at just
my level for the moment! :-)

D
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[Haskell-cafe] Re: Haskell Zippers on Wikibooks: teasing! :)

[Heinrich Apfelmus]

Peter Verswyvelen wrote:
> After my colleague explained me about zippers and how one could derive the
> datatype using differential rules, I had to read about it.
> 
> So I started reading
> http://en.wikibooks.org/wiki/Haskell/Zippers#Mechanical_Differentiation
> 
> This page contains the sentence:  *"For a systematic construction, we need
> to calculate with types. The basics of structural calculations with types
> are outlined in a separate chapter **Generic
> Programming*
> * and we will heavily rely on this material"*
> *
> *
> However, the generic programming link does not exist yet :-)

A clear case of laziness on the author's part... wait, that would be me.
:-O In any case, contributions to the wikibook would be most welcome. ;-)


For now, I'd recommend

   Generic Programming: An introduction
   http://www.cse.chalmers.se/~patrikj/poly/afp98/

It's a bit verbose at times, but you only need the first few chapters to
get an idea about polynomial functors (sums and pairs) and  mu .



Regards,
apfelmus

--
http://apfelmus.nfshost.com

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[Haskell-cafe] Re: Haskell Zippers on Wikibooks: teasing! :)

[Jon Fairbairn]

Matthias Görgens  writes:

>> doesn't make much sense to me yet, although I suspect I can read the mu as a
>> lambda on types?
>
> Not really.  The mu has more to do with recursion.

I'd say it's entirely to do with recursion. It's like the Y combinator
(or fix) for types, though it is combined with a lambda.

mu t . t is like fix (\t -> t)

-- 
Jón Fairbairn jon.fairba...@cl.cam.ac.uk
http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html  (updated 2009-01-31)

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