Improving the instances of Data.Functor.{Product,Sum}

[Henning Thielemann]

On Fri, 13 Mar 2020, chessai . wrote:

> Could you expound on why you try to avoid Flexible{Contexts,Instances} 
> in your own code? That might be useful to look at here.

I also try to avoid FlexibleInstances. FlexibleInstances are sometimes a 
consequence of multi-parameter type classes with too many parameters. E.g.

    instance Convert Int a
    instance Convert Integer a

which should be better a single-parameter type class on the first 
parameter.

FlexibleInstances might indicate too lax instances. E.g.

    instance C (A a) (B a) where

should be better

    isntance (a~b) => C (A a) (B b) where


Sometimes they are a sign of non-composability. E.g.

    instance StringClass String where

should be better

    instance CharClass a => StringClass [a] where

or

    instance (a ~ Char) => StringClass [a] where

depending, on what you need.