fromNumber :: Number -> Rational?

[hdgarrood]

Might be hard to implement, and would probably have to round sometimes? But it could be useful.

[adius]

Here is a (shitty) implementation I'm using.
Feel free to use it as inspiration to implement something decent 😛

testNumberChar :: Char -> Boolean
testNumberChar char =
  let digitArray = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '.']
  in elem char digitArray


digitsToRational :: String -> Maybe Rational
digitsToRational stringOfDigits =
  let
    isNumChar = all testNumberChar (toCharArray stringOfDigits)
  in case isNumChar of
    false -> Nothing
    true -> do
      let
        numeratorStr = replaceAll (Pattern ".") (Replacement "") stringOfDigits
        numStrLength = length numeratorStr
        index = fromMaybe numStrLength (indexOf (Pattern ".") stringOfDigits)
        denominator =  10 `pow` (numStrLength - index)
      numerator <- fromString numeratorStr
      pure (numerator % denominator)

[adius]

And of course some tests:

describe "digitsToRational" do
  it "converts 137 to 137/1" do
    (digitsToRational "137") `shouldEqual` (Just (137 % 1))

  it "converts 13 to 13/1" do
    (digitsToRational "13") `shouldEqual` (Just (13 % 1))

  it "converts 3 to 3/1" do
    (digitsToRational "3") `shouldEqual` (Just (3 % 1))

  it "converts 0 to 0/1" do
    (digitsToRational "0") `shouldEqual` (Just (0 % 1))

  it "converts 0.3 to 3/10" do
    (digitsToRational "0.3") `shouldEqual` (Just (3 % 10))

  it "converts .3 to 3/10" do
    (digitsToRational ".3") `shouldEqual` (Just (3 % 10))

  it "converts 0.300 to 3/10" do
    (digitsToRational "0.300") `shouldEqual` (Just (3 % 10))

  it "converts 2.1 to 21/10" do
    (digitsToRational "2.1") `shouldEqual` (Just (21 % 10))

  it "converts 3.21 to 321/100" do
    (digitsToRational "3.21") `shouldEqual` (Just (321 % 100))

  it "converts 12.3456 to 123456/10000" do
    (digitsToRational "12.3456") `shouldEqual` (Just (123456 % 10000))

  it "converts abc to Nothing" do
    (digitsToRational "abc") `shouldEqual` Nothing