「haskellの数値のなんか」を型クラスを使わずに

haskellの数値のなんかのコードを自分なりに書きなおしてみた。
型クラスはあまり使いこなせないので、変換関数を作ってかます方針で。 型変換の条件はわかりやすくなったと思う。

module Main where

import Data.Ratio
import Numeric

data Foo = FooInteger Integer
    | FooRational Rational
    | FooDouble Double
    deriving (Show)

addFoo :: Foo -> Foo -> Foo
addFoo (FooInteger x) (FooInteger y) =
    FooInteger $ x + y
addFoo (FooInteger x) (FooRational y) =
    FooRational $ fromInteger x + y
addFoo (FooRational x) (FooInteger y) =
    FooRational $ x + fromInteger y
addFoo (FooRational x) (FooRational y) =
    FooRational $ x + y
addFoo (FooInteger x) (FooDouble y) =
    FooDouble $ fromInteger x + y
addFoo (FooDouble x) (FooInteger y) =
    FooDouble $ x + fromInteger y
addFoo (FooRational x) (FooDouble y) =
    FooDouble $ fromRat x + y
addFoo (FooDouble x) (FooRational y) =
    FooDouble $ x + fromRat y
addFoo (FooDouble x) (FooDouble y) =
    FooDouble $ x + y

-- 以下新規コード

toD :: Foo -> Double
toD (FooInteger v) = fromInteger v
toD (FooRational v) = fromRat v
toD (FooDouble v) = v

toR :: Foo -> Rational
toR (FooInteger v) = fromInteger v
toR (FooRational v) = v

addFoo' :: Foo -> Foo -> Foo
addFoo' x@(FooDouble _) y = FooDouble $ toD x + toD y
addFoo' x y@(FooDouble _) = FooDouble $ toD x + toD y
addFoo' x@(FooRational _) y = FooRational $ toR x + toR y
addFoo' x y@(FooRational _) = FooRational $ toR x + toR y
addFoo' (FooInteger x) (FooInteger y) = FooInteger $ x + y

main :: IO ()
main = do
    let x = FooInteger 3
    let y = FooRational (3%4)
    let z = FooDouble 2.0
    print "original version"
    print $ addFoo x x
    print $ addFoo x y
    print $ addFoo x z
    print $ addFoo y y
    print $ addFoo y z
    print $ addFoo z z
    print "my version"
    print $ addFoo' x x
    print $ addFoo' x y
    print $ addFoo' x z
    print $ addFoo' y y
    print $ addFoo' y z
    print $ addFoo' z z