SillyVersion.hs

{-# Language FlexibleInstances #-}
module SillyVersion where

import Data.Ratio

data SillyVersion = SillyVersion Integer Integer Integer

display :: (() -> SillyVersion) -> String 
display f = let SillyVersion maj min pat = f ()
             in show maj <> "." <> show min <> "." <> show pat 

data Patch = Patch Integer 

version :: (Patch -> SillyVersion) -> (Patch -> SillyVersion)
version = id

instance Num (Patch -> SillyVersion) where 
  (+) = undefined
  (-) = undefined
  (*) = undefined
  negate = id 
  abs = id
  signum = id
  fromInteger = undefined

instance Fractional (Patch -> SillyVersion) where 
  fromRational r (Patch p) = 
    let num = numerator r
        den = denominator r
        (maj, rest) = num `quotRem` den
        go :: Integer -> Integer
        go 0 = 0
        go x = let (d, next) = (10 * x) `quotRem` den
               in d * 10 + go next
        min = go rest `div` 10
    in SillyVersion maj min p
  (/) = undefined 
  recip = undefined

instance Num (()-> Patch) where 
  (+) = undefined
  (-) = undefined
  (*) = undefined
  negate = id 
  abs = id
  signum = id
  fromInteger = const . Patch . fromInteger

example :: String
example = display $ version 3.4.21

-- Unfortunately, there's no way to make this work when the  minor version has trailing 0s
-- eg, this is "3.1.21
brokenExample = display $ version 3.100.21