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

Juan Casanova juan.casanova at ed.ac.uk
Fri Oct 4 21:08:32 UTC 2019

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  

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.


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