[Haskell-cafe] Re: [Haskell] ANN: Uniplate 1.0

[Isaac Dupree]

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Neil Mitchell wrote:
> Hi
> 
> Thinking about this slightly further...
> 
>> For the implementation, Data.Derive has a special case for lists,
>> tuples and Maybe. Its a shame that only a restricted number of types
>> are supported - things like Data.Map/Data.Set can be supported
>> perfectly, apart from restrictions in Template Haskell.
> 
> There are two cases.
> 
> 1) The data structure contains values, in specific places. Lists,
> tuples, Either etc. are all like this. The rules in the paper cover
> all these situations.
> 
> 2) The data structure contains values, but their place is a feature of
> the data structure - i.e. Map/Set. In this case the right thing to do
> is probably to do fromList/toList pairs on them.

Use Data.Traversable somehow - lots of things are instances of that.
I'm just reading a paper about it...

"A similar benefit can be found in the reassembly of a full data
structure from separate shape and contents. This is a stateful
operation, where the state consists of the contents to be inserted; but
it is also a partial operation, because the number of elements provided
may not agree with the number of positions in the shape."
(p.10 http://web.comlab.ox.ac.uk/oucl/work/bruno.oliveira/iteratorJFP.pdf )

That reminded me of the 'uniplate' function... I haven't thought too
closely about this, but it feels right.

Also I wonder, since we can do deriving Functor, and Traversable
instances are equally straightforward according to the paper, should we
have deriving(Traversable)?  Of course this would depend on the order of
the constructor arguments just like deriving Ord, etc, do.


Isaac
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