Report functions in let expressions that do not depend on local values

[jfmengels]

What the rule should do:

Report functions in let expressions that do not depend on local values

Prior work: elm-analyse has a rule to forbid functions in let expressions, regardless of how they are used. https://stil4m.github.io/elm-analyse/#/messages/FunctionInLet
I don't like this rule and disabled it when I was using elm-analyse because functions could be using closures in an aim to make the code using it simpler.

What problems does it solve:

Making functions much smaller in a way that makes them much more readable IMO.

I don't know if defining functions inside let expressions adds a performance cost because the functions would be redefined at every call.

Example of things the rule would report:

kebabCase str =
  let
    toLowerCase s = String.toLowerCase s
  in
  magicAlgorithm str |> toLowerCase

Example of things the rule would not report:

toLowerCase s = String.toLowerCase s

kebabCase str =
  magicAlgorithm str |> toLowerCase

func n =
  let
    -- depends on `n`
    func m = m + n
  in
  func 1

When (not) to enable this rule:

Only enable if if you care about this kind of cleanup? 🤷‍♂️

Questions I have

• Should this be applied only on functions, or also on constants?

If so, in the following example secondsInMinute should be moved to the top-level.

toSeconds nbMinutes =
  let
    secondsInMinute = 60
  in
  nbMinutes * secondsInMinute

I'd say why not, but it might be too much? Note that point-free functions look like constants, especially without type annotations

• In what package would you see this being available?

[MartinSStewart]

I don't like this rule and disabled it when I was using elm-analyse because functions could

Could what?