I want a fuction where you put in a String like "Hello I am 25 years old!" and get a list out of it like ["hello","i","am","years","old"]. So it should put all uppercase letters in lowercase and delete everything that isnt a letter. It should just use Data.List and Data.Char. I know i should use words on the String and then filter it but i just cant figure it out (yes im new to Haskell).
toString :: String -> [String]
toString str = ...
At the risk of answering a homework question:
import Data.Char
toString :: String -> [String]
toString str = filter (not . null) . map (map toLower . filter isAlpha) . words $ str
Prelude Data.Char> toString "Hello I am 25 years old!"
["hello", "i", "am", "years", "old"]
[String]' with actual type String -> [[Char]]' In the expression: filter (not . null) . map (map toLower . filter isAlpha) . words In an equation for `getWords': getWords str = filter (not . null) . map (map toLower . filter isAlpha) . words
str argument in your function definition, then you also need to pass it to the function implementation. It should be either getWords str = ... str or getWords = ...This would be my workflow: Use hoogle and work with the type signatures you need.
On how to get a list of words: You need a function foo that does "one two three" -> ["one", "two", "three"] so it has the type signature: String -> [String]. Search for exactly this type signature via hoogle and you will find the function words second in the result list.
a method which does uppercase would mostly like have the type signature Char -> Char. Type this into hoogle and you will find toUpper in third in the list.
is a letter: Char -> Bool
next open ghci and try out the functions. IE:
ghci> :t toUpper -- will print the type of isUpper
<interactive>:1:1: Not in scope: ‘isUpper’ -- you need to import Data.Char
ghci> import Data.Char -- so let´s import Data.Char
ghci> toUpper "abc"
ghci> words "a quick brown fox"
ghci> :t map
ghci> map toUpper ["a", "quick"]
... and so on
You`ll still need to figure out how to put these parts together with map and filter but again I´d advice you to have a close look at the types.
filter (not . null) . map (filter Data.Char.isLower . map Data.Char.toLower) . words. Think of it as three stages written right-to-left: break on spaces, within each word, convert to lower and prune out extras, then finally eliminate the empty words.