I am new to Haskell. I have string containing single character for example "A" or "3". I want to turn it into character to check if it is a digit with isDigit :: Char -> Bool function. How can I turn that the string into character?
I tried:
isDigit head x
-- and
isDigit take 1 x
witch gave me errors:
Variable not in scope: isDigit :: ([a0] -> a0) -> String -> Bool and Variable not in scope: isDigit :: (Int -> [a0] -> [a0]) -> t0 -> String -> Bool
The function isDigit is defined in Data.Char, so you'll need to import that module. Also, as noted in the comment, the expression isDigit head x will try to call isDigit with two arguments, head, and x. You want to evaluate head x and pass that single argument to isDigit, so you'll need to write isDigit (head x). The following program should work:
import Data.Char
string1 = "A"
string2 = "3"
main = do
print $ isDigit (head string1)
print $ isDigit (head string2)
or if you are trying to evaluate this interactively in GHCi, use:
ghci> import Data.Char
ghci> isDigit (head "A")
False
ghci> isDigit (head "3")
True
isDigit head xcallsisDigitwith two arguments (headandx). You probably wantisDigit (head x)instead. The expressionisDigit take 1 xsuffers from the same issue (but won't work anyway, sincetakereturns a list/string, not a single char).all isDigitto be robust to mistakenly calling with strings of other lengths. Or, if this is part of a validation pass, something likesingleDigit :: String -> Maybe Int; singleDigit [x] | isDigit x = Just (digitToInt x); singleDigit _ = Nothing, which returns a failed validation for other lengths, and converts to a more suitable internal representation when validation passes.