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module Main where
import Data.Char (isSpace, isPunctuation, isAlpha)
-- the sample case expression
function :: String -> String
function xs =
case (xs == "Julie") of
True -> (xs ++ " is 43.")
False -> "How old are you?"
-- if you want to play with taking user input, you can try
-- running that function with this `main`. `getLine` is an
-- IO action to get a string from user input and pass it
-- in as an argument. you could
-- write another branch that could take an answer to the
-- printed question and respond depending on various
-- answers. you needn't worry yet about what >>= is. it is
-- called `bind` and it binds the result of `getLine` to
-- the next function called; that is, it lets us pass that
-- result to another function, similar to what function
-- composition does. more on this later.
--main :: IO ()
--main = getLine >>= \name -> putStrLn (function name)
-- strip any *leading* whitespace off the input first
stripSpacePwd :: String -> Maybe String
stripSpacePwd "" = Nothing
stripSpacePwd (x:xs) =
case (isSpace x) of
True -> stripSpacePwd xs
-- is recursive to strip off as many leading
-- whitespaces/punctuations as there are
False -> Just (x:xs)
-- check to see that all characters are alphabetic
-- this will return Nothing if there are any other
-- characters in the password, including whitespace
checkAlpha :: String -> Maybe String
checkAlpha "" = Nothing
checkAlpha xs =
case (all isAlpha xs) of
False -> Nothing
True -> Just xs
-- check that the input String is 15 characters or less
validateLength :: String -> Maybe String
validateLength s =
case (length s > 15) of
True -> Nothing
False -> Just s
-- we can't do either of these because the result of each function
-- application is a `Maybe String` which cannot be passed directly
-- into the next function as a `String` argument (none of these
-- accepts a `Maybe String` as an input). these both throw type errors.
--makePwd :: String -> Maybe String
--makePwd xs = validateLength . checkAlpha . stripSpacePwd $ xs
--makePasswd :: String -> Maybe String
--makePasswd xs = validateLength (checkAlpha (stripSpacePwd xs))
validatePassword :: String -> Maybe String
validatePassword xs = case stripSpacePwd xs of
Nothing -> Nothing
Just xs' ->
case checkAlpha xs' of
Nothing -> Nothing
Just xs'' ->
case validateLength xs'' of
Nothing -> Nothing
Just xs''' -> Just xs'''
validatePassword' :: String -> Maybe String
validatePassword' xs = stripSpacePwd xs
>>= checkAlpha
>>= validateLength
main :: IO ()
-- without do syntax, using >>= (the monad is IO)
main = getLine >>= (\input -> print (validatePassword input))
mainWithDo :: IO ()
mainWithDo = do
password <- getLine
print (validatePassword password)
-- but instead of nesting all the cases into one very large function,
-- we can get that nested type of behavior while avoiding the
-- problem that we had with function composition and parenthesized
-- application if we use `bind` (>>=)
-- you can use `do` syntax with any monad; here
-- our monad is still `Maybe` but this is what
-- the above function makePassword looks like with
-- some `do` sugar on top
validatePasswordDo :: String -> Maybe String
validatePasswordDo xs = do
xs' <- stripSpacePwd xs
xs'' <- checkAlpha xs'
validateLength xs''