Parse string representation of integer in Haskell

Here it is…

	The MIT License (MIT)

	Copyright (c) 2014 Richard Cook

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	parseDigits l = parseDigits' l (10 ^ (length l - 1))
	where
	parseDigits' l multiplier
	\| null l = 0
	\| otherwise = multiplier * parseDigit (head l) + parseDigits' (tail l) (multiplier `div` 10)
	parseDigit c
	\| c >= '0' && c <= '9' = fromEnum c - fromEnum '0'
	\| otherwise = error "parseDigit: character is not a digit"

	main = do
	print (parseDigits "34567")

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