Question

I’m brand new to haskell and am working through some tutorial problems for a functional programming course I am taking. There is one problem that I am completely stumped on.

Define a function that returns whether a given string contains all numerical values or not (i.e. “123” => True, “1a3” => False). The function must use a list comprehension.

It’s this last part that is killing me. It’s easy to write without a list comprehension. It’s also easy to write a list comprehension with a predicate to ensure that you only put numerical chars into a new list.

isNum  =  [ x | x <- xs, x `elem` ['0'..'9'] ]

However I’m not sure how to then compare the new list to the original list to check for equality since this is the entire function definition.

Answer 1

Is this homework?

Since the list comprehension will only include characters that are numerals, if there are any non-numeric elements, the resulting list will be shorter.

Another approach is to put isNum xs = [ elem x ['0'..'9'] | x <- xs ]. Then you have a list of Boolean values telling you whether each character was a numeral. You can use a Prelude function to tell you whether all the values were True or not.

EDIT: more efficiently, there is also a Prelude function that can tell you whether any of the elements was False, indicating a non-numeric element.