[Haskell-cafe] Optional "Default" instances of classes in certain subcases (but too broad)

[Alexis King]

Hello,

The recent DerivingVia language extension is, in many ways, designed to accommodate the use cases you describe. Instead of defining “optional” instances, you attach instances to a relevant newtype, then use that newtype with DerivingVia to reuse instances.

So, for example, you could define a WrappedBifunctor newtype:

newtype WrappedBifunctor f a b = WrappedBifunctor { unWrappedBifunctor :: f a b }
  deriving newtype (Bifunctor)

…then declare the following instance:

instance Bifunctor f => Functor (WrappedBifunctor f a) where
  fmap = bimap id

Then if you have a type with an explicit Bifunctor instance, you can write:

data F a b = ...
  deriving (Functor) via WrappedBifunctor F

…or, with StandaloneDeriving:

deriving via (WrappedBifunctor F) instance Functor (F a)

Does that accommodate the use cases you’re looking for?

Alexis

> On Oct 4, 2019, at 16:08, Juan Casanova <juan.casanova at ed.ac.uk> wrote:
> 
> Hello again,
> 
> Before my question, I notice I am using this list quite a bit. I hope I am no abusing or misusing it by doing so, though. Sometimes I wonder what's the line between what I should ask here and what I should ask in StackOverflow, if that line even exists. If anyone feels I may be abusing or misusing, please let me know (in public or in private).
> 
> Very simple question: Would it make sense, or does it already exist, a way to implement *optional* default instances of classes that can be used more directly than currently.
> 
> I am *not* talking about providing a default implementation of a class when declaring it. I know you can do this.
> 
> I am talking about when you know you can provide a generic instance of a class for a wide range of situations *which depends on some constraints*. For example:
> 
> instance Bifunctor f => Functor (f a) where
>    fmap = bimap id
> 
> Of course, this is a terrible idea, because the constraints are not checked when verifying overlap and the like, and so this is very likely to break your program. What I tend to do nowadays is to add a function:
> 
> fmapFromBiMap :: Bifunctor f => (b -> c) -> (f a b) -> (f a c)
> fmapFromBiMap = bimap id
> 
> and then use it each time (the following is obviously a silly example):
> 
> instance Functor (Either a) where
>    fmap = fmapFromBiMap
> 
> The only problem I see with this is that I need to remember (specially with my own type classes): 1. That I provided this "default" implementation. 2. Its name. I know these are fairly small things to complain about, but once again when you do it 100 times it becomes annoying, specially for a person who absolutely does not like doing things that I know would be able to explain how to automate.
> 
> So, what I would expect is for it to be something like declaring the default instance but only use it for the types that explicitly ask for it. E.g.:
> 
> optional instance Bifunctor f => Functor (f a) where
>    fmap = bimap id
> 
> instance Functor (Either a) using optional
> 
> The main important point here is that I expect this to check the constraints in compile time for the class that I am giving (and if it contains type variables, only use those that are guaranteed to be complied, of course). This means that if I did:
> 
> instance Functor (f a) using optional
> 
> I *do not* expect it to be okay with it and automatically add the constraint Bifunctor f => to the instance. I expect it to give me a compile error: "No matching optional instance found".
> 
> Of course, there could be several optional instances that overlap when using optional. But then, GHC would give me the error indicating what the options are *and I would know that it's because I put optional* and I could just replace it with the specific one to use. Maybe by giving them names?
> 
> optional instance fmapFromBiMap Bifunctor f => Functor (f a) where
>    fmap = bimap id
> 
> instance Functor (Either a) using optional -- If this does not work because it overlaps with other optionals, then GHC tells me, indicates the options, and I replace with:
> 
> instance Functor (Either a) using fmapFromBiMap
> 
> This avoids the problem of defining the instance in general (that it will overlap with basically anything), should be easy to type check and prevents having to keep an index in your mind about useful functions that you implemented or didn't.
> 
> I wonder if the "deriving" family of functionalities have anything like this, but my search has not been fruitful. Maybe using Generics?
> 
> Maybe I just need good autocomplete so that I can find the function easily... it still feels like this would make sense.
> 
> Juan.
> 
> -- 
> The University of Edinburgh is a charitable body, registered in
> Scotland, with registration number SC005336.
> 
> 
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