string,list,haskell,recursion , Haskell Input to create a String List


Haskell Input to create a String List

Question:

Tag: string,list,haskell,recursion

I would like to allow a user to build a list from a series of inputs in Haskell.

The getLine function would be called recursively until the stopping case ("Y") is input, at which point the list is returned.

I know the function needs to be in a similar format to below. I am having trouble assigning the correct type signatures - I think I need to include the IO type somewhere.

getList :: [String] -> [String]
getList list =  do line <- getLine
                   if line ==  "Y"
                      then return list
                      else getList (line : list)

Answer:

So there's a bunch of things that you need to understand. One of them is the IO x type, which is a computer program that does something and afterwards will be able to supply you an x. So getLine doesn't do anything by itself; it just is a certain sort of program. The most fundamental way to chain such programs together is to take a program that produces an x (an IO x) and then a Haskell function which takes an x and constructs a program which produces a y (an x -> IO y and combines them together into a resulting program producing a y (an IO y.) This function is called >>= and pronounced "bind". In fact this way is universal, if we add a program which takes any Haskell value of type x and produces a program which does nothing and produces that value (return :: x -> IO x). This allows you to use, for example, the Prelude function fmap f = (>>= return . f) which takes an a -> b and applies it to an IO a to produce an IO b.

So It is so common to say things like getLine >>= \line -> putStrLn (upcase line ++ "!") that we invented do-notation, writing this as

do 
    line <- getLine
    putStrLn (upcase line ++ "!")

Notice that it's the same basic deal; the last line needs to be an IO y for some y.

Now that you know that Haskell has no magic and the Haskell IO x type just is a static representation of a computer program as a value, rather than something which does stuff when you "reduce" it (like it is in other languages), we can turn to your getList. Clearly getList :: IO [String] makes the most sense based on what you said: a program which allows a user to build a list from a series of inputs.

Now to build the internals, you've got the right guess: we've got to start with a getLine and either finish off the list or continue accepting inputs, prepending the line to the list:

getList = do
    line <- getLine
    if line == 'exit' then return []
                      else fmap (line:) getList

You've also identified another way to do it, which depends on taking a list of strings and producing a new list:

getList :: IO [String]
getList = fmap reverse (go []) where 
    go xs = do 
        x <- getLine
        if x == "exit" then return xs
                       else go (x : xs)

There are probably several other ways to do it.


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