Fancier types for labels

[jml]

It'd be nice to have a type class for labels such that I could define in my app how a certain domain-specific value got translated to a Prometheus label.

e.g.

class ToLabel a where
  toLabel :: a -> String

data MyStatus = Good | Bad | Indifferent

instance ToLabel MyStatus where
  toLabel Good = "good"
  toLabel Bad = "bad"
  toLabel Indifferent = "indifferent"

withLabel Good incCounter someMetric

But then, as I write this, I realise this probably devolves into wanting extensible records such that you can declare the type of a label at metric creation and have the type system make sure you've go the right value in the right place, which is probably going overboard.

[fimad]

While not perfect, I think you could do this today with the existing Label type class.

data MyStatus = Good | Bad | Indifferent

instance Label MyStatus where
  labelPairs (_, Good)        = ("MyStatus", "good")
  labelPairs (_, Bad)         = ("MyStatus", "bad")
  labelPairs (_, Indifferent) = ("MyStatus", "indifferent")

withLabel Good incCounter someMetric

The not perfect bit would be that you'd need to provide dummy labels while creating the counter.

someMetric = vector (Good) $ counter (Info "some_metric" "")

A real fix would probably entail making the Label class have two types, the type of the label an the types of the values.

[ocharles]

https://github.com/ocharles/prometheus-haskell/commit/70b07592ffb8e7cc847d7c15db7e94b80816d8ef is my take on what a Label class could look like.

It doesn't let you have "anonymous" labels, everything must be statically declared up front. Given the benefit of actually have much more explicit label assignment, I think it's worth it.