Re: [Haskell-cafe] Avoiding code-duplication for what is roughly fmap

On 2019-10-04 4:06 p.m., Sebastiaan Joosten wrote:

Using generics and default worked brilliantly, thanks! I didn't use higgledy, so let me know if I missed an opportunity for something. I just wanted to post my solution here. My only questions would be: isn't there a library on hackage that already does this?

There's rank2classes (http://hackage.haskell.org/package/rank2classes) but it leans on Template Haskell rather than generics.

However, anyone reading who just want to learn how to use generics, I recommend reading http://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts...., as that's basically all I needed to learn how to do this. (Nathan's link pointed me there, again: thanks!)

For getting everything to work, I added {-# LANGUAGE TypeOperators, DefaultSignatures #-} (GHC is great at telling me what to enable).

I was able to write a generic version for removing whitespace, which essentially is an fmap. To derive an instance, I can indeed just write: instance WhiteSpaced ClassItem where

The most straightforward thing to do, is to write a generic GWhiteSpaced class and then the WhiteSpaced class. Here is what I wrote: class GWhiteSpaced f where gremoveWS :: f a -> f a -- generic version of removeWS

instance GWhiteSpaced U1 where   gremoveWS U1 = U1 instance (GWhiteSpaced a, GWhiteSpaced b) => GWhiteSpaced (a :*: b) where   gremoveWS (x :*: y) = gremoveWS x :*: gremoveWS y instance (GWhiteSpaced a, GWhiteSpaced b) => GWhiteSpaced (a :+: b) where   gremoveWS (L1 x) = L1 $ gremoveWS x   gremoveWS (R1 x) = R1 $ gremoveWS x instance (GWhiteSpaced a) => GWhiteSpaced (M1 i c a) where   gremoveWS (M1 x) = M1 $ gremoveWS x instance (WhiteSpaced a) => GWhiteSpaced (K1 i a) where   gremoveWS (K1 x) = K1 $ removeWS x

class WhiteSpaced a where removeWS :: a -> a -- remove as much whitespace as possible without changing semantics default removeWS :: (GWhiteSpaced (Rep a), Generic a) => a -> a   removeWS = GHC.Generics.to . gremoveWS . from

This really helps generalize things in those cases where the data-structure changes, but the WhiteSpaced class is not the only class that follows this pattern. Naturally, I would like to: - avoid repeating the generic class for each fmap-like class (renaming method-names, simplifying expressions, etc, are all very similar functions). - and ideally remove the circular dependency, so I can put the generic classes into separate modules without getting orphaned instances for the specific ones.

I managed to do both with a single solution. First of all, I will use a phantom type to keep track of which instance to use. If you haven't seen a phantom type: it's just a convenient way of binding type variables and passing those around. I'd love to use the default one in the Prelude / RIO, but I can never find it and getting an extra dependency is not worth it, so I always end up defining one: data Phantom k = Phantom I'll use this phantom type later when I create an empty datatype (as empty as they get) whose only purpose is to denote that I'm using the whitespace function. Using Phantom types, I can unambiguously define: class FmapLike k f where gmap :: Phantom k -> f a -> f a class FmapInstance k a where gmapinstance :: Phantom k -> a -> a Note that the *only* purpose of the type variable k here, is to enable reuse: by filling in different values for k, I can instantiate whitespaces and other fmap-like functions in the same way. Otherwise, FmapLike and FmapInstance just mimick GWhiteSpaced and WhiteSpaced respectively.

My generic FmapLike function is nearly the same as my generic whitespace function, the only thing I add is passing the type variable around: instance FmapLike x U1 where   gmap _ U1 = U1 instance (FmapLike x a, FmapLike x b) => FmapLike x (a :*: b) where   gmap f (x :*: y) = gmap f x :*: gmap f y instance (FmapLike x a, FmapLike x b) => FmapLike x (a :+: b) where   gmap f (L1 x) = L1 $ gmap f x   gmap f (R1 x) = R1 $ gmap f x instance (FmapLike x a) => FmapLike x (M1 i c a) where   gmap f (M1 x) = M1 $ gmap f x instance FmapInstance x a => FmapLike x (K1 i a) where   gmap f (K1 x) = K1 $ gmapinstance f x

Now to define the WhiteSpace class as before, I just need six lines. Furthermore, these six lines can be in a separate file without creating orphaned instances: data WS class WhiteSpaced a where removeWS :: a -> a -- remove as much whitespace as possible without changing semantics default removeWS :: (FmapLike WS (Rep a), Generic a) => a -> a   removeWS = GHC.Generics.to . gmap (Phantom::Phantom WS) . from instance WhiteSpaced a => FmapInstance WS a where   gmapinstance _ = removeWS (The instance is not an orphan because WS is defined here)

Note that instead of WS, I could use any other datatype token, it doesn't have to be an empty datatype. I'm just defining it as an empty datatype to make it absolutely clear that it's not storing any data. It also provides a good place to document what WS is actually intended for. As a final touch, I'm defining: gmapGeneric = (\x -> GHC.Generics.to . gmap x . from) (this shortens removeWS a little)

So final question: is there any library that implements gmapGeneric or gmap?

Also a big thanks to everyone who helped put Generics in Haskell. I've seen helpful error messages and good documentation all the way through!

Best,

Sebastiaan

On Thu, Oct 3, 2019 at 6:41 PM Yuji Yamamoto mailto:whosekiteneverfly@gmail.com> wrote:

Use DeriveGenerics, and higgledy https://github.com/i-am-tom/higgledy (or some packages supporting higher kinded data) would help you.

2019年10月4日(金) 5:56 Sebastiaan Joosten mailto:sjcjoosten%2Bhaskelcafe@gmail.com>:

Hi all,

I'm writing a lot of code that looks like this: instance WhiteSpaced ClassItem where   removeWS (Method a b c) = Method (removeWS a) (removeWS b) (removeWS c)   removeWS (Declaration b) = Declaration (removeWS b)

Typically, all the way at the end there's an instance that deviates (sometimes the deviating instances are somewhere in the middle). I need to do this for a lot of functions, and a lot of data types, and all I'm doing here is rewriting the data-type declaration in a different syntax (except that you do not know the types of a, b and c from the above). For the sake of maintainability, I want to avoid this code-duplication and focus only on the deviating instances.

How to do better? I don't see how to use generics (in the hope of only writing 'instance WhiteSpaced ClassItem where' instead of the three lines above) for this: the types for a, b and c are all different here. Would this be easier with Template Haskell? (in the hope of only writing $(''something ClassItem) instead of the three lines above)

My main concern is maintainability, an ideal solution is either a clear one-liner or a library import (in the same way that aeson allows me to use generics or Template Haskell without needing to know much about them). Other solutions are welcome too.

Best,

Sebastiaan _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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