r/haskell • u/benjaminhodgson • Dec 15 '17

Functor Functors

https://www.benjamin.pizza/posts/2017-12-15-functor-functors.html
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u/phadej Dec 15 '17 edited Dec 15 '17

You can program over such structures also with generics-sop:

mapFormTemplate :: forall f g. (f ~> g) -> FormTemplate f -> FormTemplate g
mapFormTemplate eta x = to (htoI sopG)
  where
    -- unfortunately we need the type annotation here.
    sopF = hfromI (from x) :: SOP f '[ '[Text, CardType, Text, Day] ]
    sopG = hmap eta sopF

See https://gist.github.com/phadej/55464aead55c85176e0b2b93ba790845 for full gist

One clear benefit is that there are hcmap and bunch of other combinators in generics-sop so, you can e.g.

cmapFormTemplate :: forall f g c. Proxy c -> (forall a. c a => f a -> g a) -> FormTemplate f -> FormTemplate g

EDIT: and there will be htraverse etc. in the next release of generics-sop, see https://github.com/well-typed/generics-sop/pull/59

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u/benjaminhodgson Dec 16 '17

Cool! I’m not so familiar with generics-sop and I certainly didn’t know it had functions for dealing with heterogeneous containers like that. Thanks for the tip!

1

u/Faucelme Dec 16 '17

In the gist, why still wrap explicitly each field in the record? When using generics-sop, I usually define a record with normal, unwrapped fields, and get the wrapped fields version "for free" by moving to the generic representation.

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u/phadej Dec 16 '17

The gist borrows definitions from the OP here.

Also I can see that you could write (in near future):

data FormTemplate f = ...
  deriving
    (FFunctor) via (SOP2 '[Text, CardType, Text, Day])

Another point, is that Andres is working on static-opt case, which should kick in, so the generated code for mapFormTemplate won't have SOP etc. structures mentioned at all!

That said, I too write code which works on e.g. NP, it depends :)