CSE230 Wi14 - swizzle-v1

> import System
> import Data.Maybe
> import Data.Char

Swizzling One Character

First make a lookup table or association list of characters; each element is a list is a pair of the char and its swizzled version. Its easy enough to add caps as well, but note how we maintain readability with the where.

> code :: [(Char, Char)]
> code = smalls ++ caps 
>   where smalls   = zip ['a' .. 'z'] shuffles 
>         caps     = zip ['A' .. 'Z'] (map toUpper shuffles) 
>         shuffles = "thequickbrownfxjmpsdvlazyg"

Now, the pattern matching with case is nice, but an expert knows how to wield the standard library. In the old code, we have a default Char and the lookup will return a Maybe Char and we want to output the latter if it is not Nothing. In other words, we require a function of type Char -> Maybe Char -> Char and Hoogle is your friend.

> swizzleChar :: Char -> Char
> swizzleChar c = fromMaybe c (lookup c code) 

Swizzling One Line

To swizzle one line, we will swizzle each character on the line. You should be able to spot this as a map.

> swizzleLine :: String -> String
> swizzleLine = map swizzleChar

Swizzling Many Lines

To swizzle a file, we will first reverse the lines in the file and then swizzle each line of the file.

swizzleContent fileString = unlines (swizzleLines (reverse (lines fileString)))

That looks a bit ugly doesn’t it. Since we are just gluing (ie pipelining) together three operations, the intermediate variables are somewhat pointless, indeed, we can eliminate them altogether. Recall that the infix compose function

(.) f g x = f (g x)

Now, you see why compose is useful, we can define

swizzleContent fileString = (unlines . swizzleLines . reverse . lines) fileString

and further simplify it by eliminating the parameter altogether (via partial application)

swizzleContent = unlines . swizzleLines . reverse . lines

Now, of course, the helper swizzleLines is merely a map over swizzleLine

swizzleLines = map swizzleLine

Think of map as a magic wand that enables a function (eg swizzleLine) to operate over lists. Thus, we can just use equational reasoning and boil swizzleContent down to

> swizzleContent = unlines . map swizzleLine . reverse . lines

I defy you to describe swizzleContent more elegantly than that!

Doing the IO

Of course, its all very well to manipulate strings, but in the end, the rubber must hit the road. The next function takes a filename as input and returns an action that corresponds to the swizzling of the file.

> swizzleFile :: FilePath -> IO ()
> swizzleFile f = do d  <- readFile f
>                    writeFile (f ++ ".swz") (swizzleContent d) 

Hmm, it would be nice to be able to swizzle many files at one shot. Can you find a way to use patterns to eliminate the explicit recursion altogether?

swizzleFiles :: [FilePath] -> IO ()
swizzleFiles []     = return ()
swizzleFiles (f:fs) = do swizzleFile f 
                         swizzleFiles fs

Actually, there are two different things that are intermingled in the above code. First, we are creating the individual actions for each file. Second, we are stitching together the individual actions. So we can rewrite the code as

swizzleFiles :: [FilePath] -> IO ()
swizzleFiles fs = fuseActions (map swizzleFile fs) 

Now, this is still improvable. Never rewrite when you can reuse! Recall that

fuseActions :: [IO a] -> IO ()

When you query Hoogle with that type you find the (aptly named) library function sequence_ that does exactly the same thing! So.

swizzleFiles fs = sequence_ (map swizzleFile fs) 

And by now, you should be able to spot the compose pattern (by the ugly parens!) So really the code is just

> swizzleFiles = sequence_ . map swizzleFile

Finally, not much more to do here

> main = do files <- getArgs 
>           swizzleFiles files

Note that except for the very end, the code is completely pure. Merely by inspecting the function’s type we can know that it doesnt so much as breathe on the filesystem.

Now of course, there is a gigantic problem, we have code for swizzling, but not unswizzling!! Lets see how to fix that.