WIP: tree shaking of type class instances

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#4 needs to be resolved before merging this issue

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gn :: forall a. Show a => a -> Maybe String
gn a =
  -- no type annotation is present for gnRes in CoreFn for `gnRes`
  let gnRes = fn a
  in gnRes

Then the CoreFn gnRes is:

gnRes
  App
    (App
      (App
        (Var Let.fn :type ForAll m (ForALl a (ConstrainedType Data.Show.Show (ConstrainedType Control.Monad.Monad ...))))
        (Var dictShow))
      (Data.Maybe.monadMaybe))
    (Var a)

PureScript compiler places the resolved type class constrain inside the call to fn :: forall a m :: Show a=> Monad m => a -> m String. The same happens when gn looks like

gn' :: forall a. Show a => a -> Maybe String
gn' a =
  -- no type annotation is present in CoreFn for `coercedFn`
  let coercedFn x = fn x
  in coercedFn a

It seems that this is the rule that PureScript fills constraints, rather than creating constraint types for coercedFn that resolves type class instances when calling coercedFn a. But one can force this with type annotations:

gn'' :: forall a. Show a => a -> Maybe String
gn'' a =
  let 
      -- the type signature is reflected in CoreFn annotation
      coercedFn :: forall m. Monad m => a -> m String
      coercedFn x = fn x
  in coercedFn a