The error lies here:
toDigits' acc number = ((number `mod` 10 ): acc) (toDigits' (number `mod`
10))
It should instead be
toDigits' acc number = (number `mod` 10) : (toDigits' acc (number `mod` 10))
My suggestion would be to look at it like a fold.
toDigits' :: Integral a => a -> a -> [a]
toDigits' acc 0 = [acc]
toDigits' acc n = n `mod` 10 : toDigits' acc (n `div` 10)
Now this gives the digits in the reverse order, so in toDigits, you can
reverse it.
A good exercise would now be to re-write this as a fold. Graham Hutton has
a good paper about it. [1]
The best way would be to directly convert the number to a string using
show, but that's not the point of the exercise.
[1]: https://www.cs.nott.ac.uk/~gmh/fold.pdf
On 23 May 2015 at 12:28, Roelof Wobben
Hello,
For some reasons my file's are corrupted. I had repair them with success except this one.
Here is the code :
toDigits number | number <= 0 = [] | otherwise = toDigits' [] number where toDigits' acc 0 = acc toDigits' acc number = ((number `mod` 10 ): acc) (toDigits' (number `mod` 10))
main = print $ toDigits 123
and here is the error:
Couldn't match expected type `(a0 -> [a0]) -> [a0]' with actual type `[a0]' The function `(number `mod` 10) : acc' is applied to one argument, but its type `[a0]' has none In the expression: ((number `mod` 10) : acc) (toDigits' (number `mod` 10)) In an equation for toDigits': toDigits' acc number = ((number `mod` 10) : acc) (toDigits' (number `mod` 10))
Roelof
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