[10 points] Write a routine isAllDigits str that returns True iff the string is nonempty and all the characters in the string are decimal digits. You can use the Data.Char function isDigitc to see if a character is a digit. You can write your function recursively if you want, but it's not required- check out the Data.List routines map and and. (In ghci, you'll want to use :m Data.Char Data.List)