haskell - Luhn algorithm implementation -


i'm expecting luhn 5594589764218858 = true false

-- last digit number lastdigit :: integer -> integer lastdigit 0 = 0 lastdigit n = mod n 10  -- drop last digit number droplastdigit :: integer -> integer droplastdigit n = div n 10  torevdigits :: integer -> [integer] torevdigits n   | n <= 0    = []   | otherwise = lastdigit n : torevdigits (droplastdigit n)  -- double every second number in list starting on left. doubleeveryother :: [integer] -> [integer] doubleeveryother []       = [] doubleeveryother (x : []) = [x] doubleeveryother (x : y : z) = x : (y * 2) : doubleeveryother z  -- calculate sum of digits in every integer. sumdigits :: [integer] -> integer sumdigits []          = 0 sumdigits (x : [])    = x sumdigits (x : y)     = (lastdigit x) + (droplastdigit x) + sumdigits y  -- validate credit card number using above functions. luhn :: integer -> bool luhn n   | sumdigits (doubleeveryother (torevdigits n)) `div` 10 == 0 = true   | otherwise                                                  = false

i know can done easier i'm following haskell introductory. think problem in luhn function. course mentions problems may occur because torevdigits reverses number think should work anyways.

the snippet x `div` 10 == 0 not correct check x divisible ten; should use `mod` instead. also, equation incorrect:

sumdigits (x : [])    = x

(try, e.g. sumdigits [10].) can fixed, deleting simpler , still correct.


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