Re: [Haskell-cafe] Replace data constructors via meta programming

Vilem, you may have missed the post from the catamorphisms author where he updated the library in response to this thread? :) On Feb 12, 2018 3:58 PM, "Vilem-Benjamin Liepelt" wrote:

Thank you for your great suggestions.

The type-foo looks very cool and I will have to dig more into the different options.

Generating a catamorphism using the library of the same name works like a charm and integrates well with my existing code—once I managed to get it to install (thank you Stack LTS!) it just worked out of the box and let me write a one-line evaluator and a one-line pretty-printer. Woohoo!

Unfortunately It's not really viable to use this "for real" at the moment because it requires such an old version of GHC.

Something I miss is the clear correspondence between data constructors and "substitutions" (like in my `magic` example), since they become entirely positional, leading to potentially brittle code (imagine reordering the data constructors). I think some of the other solutions might be better in this respect.

It's a shame that the catamorphism package doesn't work for a more up-to-date version of GHC, because I think I would use this quite often.

I suppose the fold that Sergey proposed is essentially what the catamorphism package generates. Although I want to avoid having to write functions by hand when really the computer should be doing them for me, I think I will use this for now as it integrates nicely with my existing code and leads to quite idiomatic Haskell.

I will definitely check out the other suggestions as well though, thank you again.

Best,

Vilem

...

On 2018-02-12, at 09:08, Frerich Raabe wrote:

On 2018-02-12 03:30, Vilem-Benjamin Liepelt wrote:

...

I am looking for a solution to get rid of this silly boilerplate: eval :: Ord var => Map var Bool -> Proposition var -> Bool eval ctx prop = evalP $ fmap (ctx Map.!) prop where evalP = \case Var b -> b Not q -> not $ evalP q And p q -> evalP p && evalP q Or p q -> evalP p || evalP q If p q -> evalP p ==> evalP q Iff p q -> evalP p == evalP q

[..]

You might benefit from the 'catamorphism' package:

https://hackage.haskell.org/package/catamorphism-0.5.1.0/ docs/Data-Morphism-Cata.html

It provides a template Haskell function which, given a data type, produces a function which reduces (folds) that data type.

-- Frerich Raabe - raabe@froglogic.com www.froglogic.com - Multi-Platform GUI Testing

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