This is a quasi-literate version of the very simple URL shortener I wrote in Haskell with Scotty.
This is a literate respin of an older post of mine
First we declare our module name to be
Main as that is required for anything exporting a
main function to be invoked when the executable runs.
Next we have a series of imports. Where we import something
qualified...as, we are doing two things: qualifying the import means that we can only refer to values in the module with the full module path. One example of that would be
Control.Monad.mapM, that’s a fully qualified reference to
mapM. Second, we use
as to give the values that we want in scope a name. We qualify the import so that we don’t import values that would conflict with values that already exist in Prelude. By specifying a name using
as, we can give the value a shorter, more convenient name. Where we import the module name followed by parentheses, such as with
liftIO, we are saying we only want to import the functions or values of that name and nothing else. In the case of
import Web.Scotty, we are importing everything
Web.Scotty exports. An unqualified and unspecific import should be avoided except in cases where it will be very obvious where a function came from or because it’s a toolkit you must use all together like Scotty.
import Control.Monad (replicateM) import Control.Monad.IO.Class (liftIO) import qualified Data.ByteString as BS import qualified Data.ByteString.Lazy as BL import qualified Data.ByteString.Char8 as BC import qualified Data.List.NonEmpty as NE import Data.Semigroup import Data.Text.Encoding (decodeUtf8, encodeUtf8) import qualified Data.Text.Lazy as TL import qualified Database.Redis as R import Network.URI (parseURI) import qualified System.Random as SR import Web.Scotty
Next we need to generate our shortened URLs that will refer to the links people post to the service.
alphaNum is a
String of the characters we want to select from - an alphabet if you will.
randomElement lets us get a random element from a non-empty list. It first gets the length of the list to determine what range it is selecting from, then gets a random number in that range using IO to handle the randomness. Nota bene - I could have written
randomElement to return IO (Maybe a) instead of demanding a
NonEmpty; however, I prefer to be more clear about my inputs if that cleans up my outputs. Being more clear about my inputs also provides the user more information than “maybe I won’t give you anything” does.
Here we apply
alphaNum to get a single random letter or number from our alphabet. Then we use
replicateM 7 to repeat this action 7 times, giving a list of 7 random letters or numbers. For additional fun, see what
replicateM 2 [1, 3] does and see if you can figure out why.
saveURI, we pass our connection to Redis, the key we are setting in Redis, and the value we are setting the key to as arguments. We also perform side effects in IO to get
Either R.Reply R.Status. The key in this case is the randomly generated URI we created and the value is the URL the user wants the shortener to provide at that address. Redis is a key-value datastore which can be very convenient for some common use-cases like caching…or when you want persistence without a lot of ceremony, as was the case here.
In the case of
getURI, we just pass it the connection to Redis and the shortened URI key so that we can get the URI associated with that short URL and show the users where they’re headed.
main is a function like any other. Here it returns
IO () and acts as the entry point for our webserver when we start the executable. We begin by invoking
scotty 3000, a helper function from Scotty which, given a port to run on and a Scotty application, will listen for requests and respond to them.
Outside of any specific route handlers, we are cheating a bit and binding the database connection to Redis to the name
R.defaultConnectInfo points at an instance of Redis on my local machine, and
liftIO lifts the IO action into the Scotty monad.
liftIO is a typeclass method from MonadIO; monads which implement it can have IO actions lifted over them. Writing a trivial instance of this can be a good way to get a feel for how this works and why it’s desirable. Briefly - it saves us a bunch of
lift . lift . lift etc.
get has type
RoutePattern -> ActionM () -> ScottyM (). It takes a RoutePattern to decide which request paths to dispatch on and an action to perform when a request matches that path. It (purely) modifies the enclosing
ScottyM to register itself.
Here we use
param to get the input parameter “uri”. Where
param gets its parameters depends on the HTTP method of the action. With a
get it will check the HTTP get arguments, whereas with a
post it will check the POST form body. In all cases, it will check for parameters in the URI itself, such as with “/my/uri/:argument”.
Since we want to reject things that aren’t URIs or might’ve been accidentally malformed, we’re using
parseURI which has type
String -> Maybe Network.URI.URI to discriminate between bad and good URIs. We are using
TL.unpack because param returned a lazy Text object and not the
String parseURI expects, so we unpack it into a
String. Then we case match on the
Nothing case of the
parseURI returns to handle the “yep, a proper URL” and “nope, not a proper URL” cases separately.
Here we match on the
Just constructor of Maybe and throw away the contents because we only used
parseURI to check if the URI was correct; we don’t actually care to use the parsed URI object.
Now we call on our old friend
shortyGen to get a randomized URI to assign a shortened URI for the input URL the user gave us. Once again,
IO so we must lift that over our ActionM. Previously, we were lifting over ScottyM. Both, conveniently, implement MonadIO. One of the nice things about liftIO is that we don’t have to care how many monads the IO action got lifted over. This can save some maintenance and headache down the road.
We use a
let expression with
let shorty = BC.pack shawty because it’s not participating in the enclosing monad. It just packs the
String into a
Here we’re encoding the uri the user passed us as a
ByteString as that’s what the Redis client library wants. Before it can be encoded, we have to change it from a lazy to a strict Text object. We’re using the Redis connection in scope from earlier and the shortened URI ByteString to save the user’s data at that key in the Redis database. We lift the IO action into the Action monad transformer and bind over the result in ActionT. The response has type (after monadic binding)
Either R.Reply R.Status.
resp which is
Either R.Reply R.Status. It’ll be
Right Ok ...yadda yadda if it succeeded,
Left ... if it failed. We concatenate the stringified response with some additional text to show the user what the shortened URI is.
Now we need to handle the
Nothing case from earlier. The
Nothing case happens if
parseURI was given something that wasn’t a correct URL. We just return a text response saying the uri provided wasn’t a url.
This is another http get request handler, but this time with a parameterized URL component which has the name
short. This is so a request against
/EFG3YLB will parse
EFG3YLB as the param
param does a bit of magic for us.
param checks multiple sources of arguments for us. Where previously we were getting a parameter from the HTTP GET arguments, this time it’s a component of the request path.
Here we lift an IO action into ActionT. This time we’re taking the shortened URI path that we got and asking Redis if it has a matching value for us to give back to the user at that key. We bind over the result, giving us
uri which has type
Either R.Reply (Maybe BC.ByteString).
Next we case match on
uri over the
Right constructors of the Either sum type. If we get
Left…well, we’re done. We show the user what Redis had to say about it and give up. If we got
Nothing that means Redis didn’t have a value saved at the key we queried - nothing stored at that location. In that case, we return the text to the user “uri not found”.
If we got
Right, we’re going to proceed.
We proceed because the
Right value means the Either value was a “success” idiomatically speaking. Here
mbBS is the
Maybe BC.ByteString that was inside of
Either R.Reply (Maybe BC.ByteString).
If we got
Just, we take the
ByteString Redis returned and pack it into an HTML anchor tag so it is a clickable link for the user to follow. If that happened, the request was a success and we’re done. We use the
html instead of
text Scotty function to indicate the content type we’re returning is HTML and not plain text.