I am trying to split a string in to a list of characters and removing any non-letters in the process. Here is my code so far:
getCharList :: String -> [String]
getCharList x = [filter isLetter c | c <- splitOn "" x, c /= ""]
The output I receive from, for example:
getCharList "Why doesn't this work"
is:
["W","h","y","","d","o","e","s","n","","t","","t","h","i","s","","w","o","r","k"]
Could someone please explain why this doesn't seem to be able to remove the empty characters?
For your direct question: I will simply point out that filter isLetter c may be the empty list even when c is not the empty list; hence c /= "" does not ensure filter isLetter c /= "".
Popping up a level, I find your existing code to have a few oddities. The main thing you should realize is that String is exactly the same type as [Char] (it is just another name for it). There's no need to do any processing to convert a String into a list of its elements -- splitOn "" is basically unneeded work. The literal "foo" is just shorthand for the literal ['f', 'o', 'o'] (try them both in ghci!). I therefore propose a complete rewrite of your function, along the following lines:
getLetters :: String -> [Char]
getLetters = filter isLetter
(I used [Char] instead of String in the return type to emphasize that I intend to think of the returned thing as a list of letters rather than as a reasonable string. This is a human-level difference only; GHC would be just as happy to call the returned value a String.) This is perfectly idiomatic. If you prefer, you can also include the argument, writing instead:
getLetters s = filter isLetter s
