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 <r.wobben@home.nl> wrote:

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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Regards

Sumit Sahrawat