Deduce Type from Parameter? GHC extension?

Histogram.hs

{-# LANGUAGE RankNTypes #-}
module Histogram where

-- Hide Prelude due to VegaLite conflicts
import Graphics.Vega.VegaLite as VL
import Prelude hiding (filter, lookup, repeat)

class (Show a, Ord a, Eq a, Enum a, Bounded a) => VegaEncodable a

-- Requires RankNTypes
type CountMap a = (VegaEncodable a) => a -> Double

toHistogram :: CountMap a -> VegaLite
toHistogram v = _ -- is there a way to deduce the type of v?
-- so that I can for example do map [minBound::${type of v}..maxBound::${type of v}]


-- Part 2, is the type constraint in CountMap not enough to infer `a` is VegaEncodable?
-- this compiles
f :: (VegaEncodable a) => CountMap a -> [Double]
f cm = map cm [minBound..maxBound]

-- but this does not
f' :: CountMap a -> [Double]
f' cm = map cm [minBound..maxBound]