Why the functional dependency in *Action classes?

[wolverian]

I'd like to be able to write a calculation (a *. b) *. c where a :: A, b :: B and c :: C.

I defined instances MultiplicativeAction A B and MultiplicativeAction A C. Now a *. b :: A and denote that with a', and a' *. c :: A, the types seem to match up. However, now I get the error Functional dependencies conflict between instance declarations (as they do).

There's surely a reason why the fundep is there, but I couldn't find the reason in the Haddocks, the code or the Git history. Could you open this up a bit?

[tonyday567]

The main reason for fundeps was to avoid TypeFamilies. There has, at least historically, been a reluctance to introduce type families to Prelude due to various performance issues that I think remain unresolved to date eg

https://gitlab.haskell.org/ghc/ghc/-/issues/8095
https://gitlab.haskell.org/ghc/ghc/-/issues/12088

Back in the day, numhask mirrored this conservatism in a naive attempt for a Num redesign to get taken seriously.

The typefam branch includes a switch to TypeFamilies for a MultiplicativeAction, by including a new type, Scalar eg

-- | Multiplicative Action
class
  (Multiplicative (Scalar m)) =>
  MultiplicativeAction m
  where
  type Scalar m :: Type

  infixl 7 .*
  (.*) :: Scalar m -> m -> m

(*.) = flip (.*)

Would this work for your use case?

I'll be releasing this soon as 0.11.

[wolverian]

Thanks, I’ll try the branch!

[tonyday567]

I released 0.11 which goes with (|) rather than (.) etc. The dot was confusing some formatters and . is a way overused symbol in haskell. I hope it makes sense for your usage.