Proposal: Num instance for (a -> b)

[Henning Thielemann]

On Sat, 17 Nov 2018, Anthony Clayden wrote:

> > relevant reddit comment thread: ...
> 
> Are you people completely nuts? Haven't you wreaked enough havoc with the Foldable Traversable Piffle?

I am still using at most GHC-7.8.4 as main compiler because of broken type 
safety starting from GHC-7.10. Breaking type safety for everyone was 
achieved quickly, but restoring at least a bit of it for the ones who care 
will certainly not happen for years (presumedly not before more Wats are 
implemented):
    https://ghc.haskell.org/trac/ghc/ticket/11796

However, I cannot see how this relates to the Foldable desaster or to the 
Num (a -> b) issue:

> Why ref pointy-headed discussion on reddit when you could also ref far more frequent complaints:
> https://blog.plover.com/prog/haskell/type-errors.html
> "I think there must be something really wrong with the language design here. I don't know exactly what it is, but
>  I think someone must have made the wrong tradeoff at some point."

Since he already defined 'adj' as top-level function, I'd suggest he gives 
it a monomorphic type signature and all type ambiguities are gone. 
Top-level type signatures are good style anyway.