[Haskell-cafe] Pulling an applicative/apply out of a record

adam vogt vogt.adam at gmail.com
Mon Feb 8 23:19:24 UTC 2016

On Sun, Feb 7, 2016 at 2:25 AM, Tyson Whitehead <twhitehead at gmail.com>

> Quick question for the experts out there.
> I need to process some records.  The records come in in an unordered
> stream of record fields.  From each stream of fields I want to produce a
> Record entry containing the fields I need to do my calculation.
> data Record = { field1 :: Type1, field2 :: Type2, ... }
> I was thinking this setup might work nice with something like
> data RecordF d = RecordF { field1 :: f Type1, field2 :: f Type2, ... }
> as I could then have (1) a monoid version I could accumulate use as an
> accumulator over the stream
> type RecordM = RecordF []
> and (2) a final version.
> type Record = RecordF Identity
> This final version seems like it should easily come from the later by
> pulling the [] (or anything else that has the Apply/Applicative structure)
> to the outside and then taking the head of the resulting list.
> While I could write the boilerplate to do this, it seems like something I
> should be able to do more elegantly.  The best I have come up with so far
> is to use the lens package to give me an isomorphism to a tuple.  Then I
> need a uncurried zip for arbitrarily large tuples.  Follow this by an
> isomorphism back.
> RecordF (f Type1) (f Type2) ... -> (f Type1, f Type2, ....)
> -> f (Type1, Type2, ...) -> f (Record Type1 Type2 ...)
> I can't seem to find any uncurried zip implementations for arbitrarily
> large tuples (or any other way to do this without writing the
> boilerplate).  Am I missing something?  Is there a better way to do this?
> Thanks!  -Tyson
> PS:  It seems the Data.Vinyl package has something along these lines with
> the rtraverse function
> https://hackage.haskell.org/package/vinyl-0.5.1/docs/Data-Vinyl-Core.html
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https://gist.github.com/aavogt/f7d875abaf504dc3bf12 shows how you can do it
with extensible, HList, and vinyl.

Or if you want to stick with your current way of defining records, I think
https://wiki.haskell.org/Template_Haskell#.27generic.27_zipWith could work

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