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rtperson on 06/01/11

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Real World Haskell Exercise - Ch 4

/ Published in: Haskell

The first fold exercise from RWH, Chapter 4

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{- Ex 1: Use a fold (choosing the appropriate fold will 
    make your code much simpler) to rewrite and improve upon 
    the asInt function from the section called â€œExplicit recursionâ€.
    
    Your function should behave as follows.
    
    ghci> asInt_fold "101"
    101
    ghci> asInt_fold "-31337"
    -31337
    ghci> asInt_fold "1798"
    1798
    
    Extend your function to handle the following kinds of exceptional conditions by calling error.
    
    ghci> asInt_fold ""
    0
    ghci> asInt_fold "-"
    0
    ghci> asInt_fold "-3"
    -3
    ghci> asInt_fold "2.7"
    *** Exception: Char.digitToInt: not a digit '.'
    ghci> asInt_fold "314159265358979323846"
    564616105916946374
-}
import Data.Char (digitToInt) -- we'll need ord
import Data.List (foldl')

-- powTen was originally a one-liner. I'm trying to guard against an int overflow,
-- but haven't yet found a way to successfully do so.
asInt_fold :: String -> Int
asInt_fold []  = error "String is empty"
asInt_fold "-" = error "dash is not a number"
asInt_fold ('-':xs) = (-1) * asInt_fold xs 
asInt_fold str = foldl' powTen 0 str
                         where maxis = maxBound::Int
                               powTen n ch 
                                  | n > (maxis) = error "overflow of int"
                                  | otherwise = n * 10 + (digitToInt ch)

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