First working version

.gitignore

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+dist*/
+.ghc.environment.*

CHANGELOG.md

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+# Revision history for RomanNum
+
+## 0.1.0.0 -- 2019-11-08
+
+* A first working version, exposing a constructor, sum and shortcut for conversion from String

RomanNum.cabal

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+cabal-version:       2.4
+-- Initial package description 'RomanNum.cabal' generated by 'cabal init'.
+--   For further documentation, see http://haskell.org/cabal/users-guide/
+
+name:                RomanNum
+version:             0.1.0.0
+-- synopsis:
+-- description:
+-- bug-reports:
+-- license:
+license-file:        LICENSE
+author:              Tissevert
+maintainer:          tissevert+devel@marvid.fr
+-- copyright:
+-- category:
+extra-source-files:  CHANGELOG.md
+
+library
+  exposed-modules:     Data.RomanNum
+  -- other-modules:
+  -- other-extensions:
+  build-depends:       base ^>=4.11.1.0
+                     , containers
+  hs-source-dirs:      src
+  default-language:    Haskell2010
+  ghc-options:         -Wall

Setup.hs

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+import Distribution.Simple
+main = defaultMain

src/Data/RomanNum.hs

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+module Data.RomanNum (
+      RomanNum(..)
+    , nullus
+    , plus
+    , r
+  ) where
+
+import Data.Map (Map, (!))
+import qualified Data.Map as Map (fromList)
+
+newtype RomanNum = RomanNum Int
+
+data RomanChar = I | V | X | L | C | D | M deriving (Bounded, Eq, Ord, Enum, Show, Read)
+
+romanChars :: [RomanChar]
+romanChars = [minBound .. maxBound]
+
+base :: [(Int, [Char])]
+base = base' 1 [] . concat $ show <$> romanChars where
+  base' _ l "" = l
+  base' size l chars =
+    base' (size * 10) ((size, take 3 chars):l) (drop 2 chars)
+
+values :: Map RomanChar Int
+values = Map.fromList $
+  zip romanChars (concat $ iterate (fmap (*10)) [1, 5])
+
+instance Show RomanNum where
+  show (RomanNum 0) = "nullus"
+  show (RomanNum n) = show' n base where
+    show' 0 _ = ""
+    show' _ [] = error "Numeral system not fine-grained enough"
+    show' k ((size, digits):others)
+      | k < size = show' k others
+      | otherwise =
+        let decDigit = k `div` size in
+        showDecDigit decDigit digits ++ show' (k `mod` size) others
+    showDecDigit _ [] = error "no digits for conversion"
+    showDecDigit k [digit] = k `times` digit
+    showDecDigit k digits
+      | k == 4 || (k == 9 && length digits > 2) =
+        (head digits) : [head (drop (if k > 5 then 2 else 1) digits)]
+      | k > 4 = head (drop 1 digits) : (showDecDigit (k-5) digits)
+      | otherwise = k `times` (head digits)
+    times count letter = take count $ repeat letter
+
+r :: String -> RomanNum
+r "" = nullus
+r "nullus" = nullus
+r n@[_] = RomanNum (values ! (read n))
+r (digit:nextDigit:otherDigits) =
+  let (rChar, nextRChar) = (read [digit], read [nextDigit]) in
+  if rChar < nextRChar
+  then RomanNum (values ! nextRChar - values ! rChar) `plus` r otherDigits
+  else RomanNum (values ! rChar) `plus` r(nextDigit:otherDigits)
+
+nullus :: RomanNum
+nullus = RomanNum 0
+
+plus :: RomanNum -> RomanNum -> RomanNum
+plus (RomanNum a) (RomanNum b) = RomanNum (a + b)