Ch24_Fractions.hs

{-# OPTIONS_GHC -fno-warn-missing-signatures #-}
{-# LANGUAGE OverloadedStrings #-}

module Text.Fractions where

import Control.Applicative
import Data.Attoparsec.Text (parseOnly)
import Data.Ratio ((%))
import Data.String (IsString)
import Text.Trifecta

badFraction :: IsString s => s
badFraction = "1/0"

alsoBad :: IsString s => s
alsoBad = "10"

shouldWork :: IsString s => s
shouldWork = "1/2"

shouldAlsoWork :: IsString s => s
shouldAlsoWork = "2/1"

parseFraction :: (Monad m, TokenParsing m) => m Rational
parseFraction = do
  numerator <- decimal
  char '/'
  denominator <- decimal
  return (numerator % denominator)

parseFraction' :: (Monad m, TokenParsing m) => m Rational
parseFraction' = do
  numerator <- decimal
  char '/'
  denominator <- decimal
  case denominator of
    0 -> fail "Denominator can't be zero"
    _ -> return (numerator % denominator)

main :: IO ()
main = do
  -- parseOnly is Attoparsec
  print $ parseOnly parseFraction' badFraction
  print $ parseOnly parseFraction' shouldWork
  print $ parseOnly parseFraction' shouldAlsoWork
  print $ parseOnly parseFraction' alsoBad

  -- parseString is Trifecta
  print $ parseString parseFraction' mempty badFraction
  print $ parseString parseFraction' mempty shouldWork
  print $ parseString parseFraction' mempty shouldAlsoWork
  print $ parseString parseFraction' mempty alsoBad