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module Base10Integer where
import Control.Applicative
import Text.Trifecta
-- 2. Write a parser for positive integer values. Don't reuse the preexisting
-- digit or integer functions, but you can use the rest of the libraries we've
-- shown you so far. You are not expected to write a parsing library from
-- scratch.
--
-- Hint: Assume you're parsing base-10 numbers. Use arithmetic as a cheap
-- "accumulator" for your final number as you parse each digit left-to-right.
parseDigit :: Parser Char
parseDigit = (char '0')
<|> (char '1')
<|> (char '2')
<|> (char '3')
<|> (char '4')
<|> (char '5')
<|> (char '6')
<|> (char '7')
<|> (char '8')
<|> (char '9')
charToDigit :: Char -> Integer
charToDigit c = case c of
'0' -> 0
'1' -> 1
'2' -> 2
'3' -> 3
'4' -> 4
'5' -> 5
'6' -> 6
'7' -> 7
'8' -> 8
'9' -> 9
base10Integer :: Parser Integer
base10Integer = do
number <- some parseDigit
let n = foldl (\acc x -> (acc * 10) + (charToDigit x)) 0 number
return n
-- 3. Extend the parser your wrote to handle negative and positive integers.
-- Try writing a new parser in terms of the one you already have to do this.
base10Integer' :: Parser Integer
base10Integer' = do
negative <- (char '-' >> (return negate)) <|> (return id)
number <- some parseDigit
let n = foldl (\acc x -> (acc * 10) + (charToDigit x)) 0 number
return $ negative n
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