Compojure explained (to some degree)
NB. I am working with Compojure 0.4.1 (here's the 0.4.1 release commit on GitHub).
Why?
At the very top of compojure/core.clj, there's this helpful summary of Compojure's purpose:
A concise syntax for generating Ring handlers.
On a superficial level, that's all there is to the "why" question. To go a bit deeper, let's have a look at how a Ring-style app functions:
A request arrives and is transformed into a Clojure map in accordance with the Ring spec.
This map is funnelled into a so-called "handler function", which is expected to produce a response (which is also a Clojure map).
The response map is transformed into an actual HTTP response and sent back to the client.
Step 2. in the above is the most interesting, as it is the handler's responsibility to examine the URI used in the request, examine any cookies etc. and ultimately arrive at an appropriate response. Clearly it is necessary that all this work be factored into a collection of well-defined pieces; these are normally a "base" handler function and a collection of middleware functions wrapping it. Compojure's purpose is to simplify the generation of the base handler function.
How?
Compojure is built around the notion of "routes". These are actually implemented at a deeper level by the Clout library (a spinoff of the Compojure project -- many things were moved to separate libraries at the 0.3.x -> 0.4.x transition). A route is defined by (1) an HTTP method (GET, PUT, HEAD...), (2) a URI pattern (specified with syntax which will apparently be familiar to Webby Rubyists), (3) a destructuring form used in binding parts of the request map to names available in the body, (4) a body of expressions which needs to produce a valid Ring response (in non-trivial cases this is usually just a call to a separate function).
This might be a good point to have a look at a simple example:
(def example-route (GET "/" [] "<html>...</html>"))
Let's test this at the REPL (the request map below is the minimal valid Ring request map):
user> (example-route {:server-port 80
:server-name "127.0.0.1"
:remote-addr "127.0.0.1"
:uri "/"
:scheme :http
:headers {}
:request-method :get})
{:status 200,
:headers {"Content-Type" "text/html"},
:body "<html>...</html>"}
If :request-method were :head instead, the response would be nil. We'll return to the question of what nil means here in a minute (but notice that it is not a valid Ring respose!).
As is apparent from this example, example-route is just a function, and a very simple one at that; it looks at the request, determines whether it's interested in handling it (by examining :request-method and :uri) and, if so, returns a basic response map.
What is also apparent is that the body of the route does not really need to evaluate to a proper response map; Compojure provides sane default handling for strings (as seen above) and a number of other object types; see the compojure.response/render multimethod for details (the code is entirely self-documenting here).
Let's try using defroutes now:
(defroutes example-routes
(GET "/" [] "get")
(HEAD "/" [] "head"))
The responses to the example request displayed above and to its variant with :request-method :head are like expected.
The inner workings of example-routes are such that each route is tried in turn; as soon as one of them returns a non-nil response, that response becomes the return value of the whole example-routes handler. As an added convenience, defroutes-defined handlers are wrapped in wrap-params and wrap-cookies implicitly.
Here's an example of a more complex route:
(def echo-typed-url-route
(GET "*" {:keys [scheme server-name server-port uri]}
(str (name scheme) "://" server-name ":" server-port uri)))
Note the destructuring form in place of the previously used empty vector. The basic idea here is that the body of the route might be interested in some information about the request; since this always arrives in the form of a map, an associative destructuring form can be supplied to extract information from the request and bind it to local variables which will be in scope in the route's body.
A test of the above:
user> (echo-typed-url-route {:server-port 80
:server-name "127.0.0.1"
:remote-addr "127.0.0.1"
:uri "/foo/bar"
:scheme :http
:headers {}
:request-method :get})
{:status 200,
:headers {"Content-Type" "text/html"},
:body "http://127.0.0.1:80/foo/bar"}
The brilliant follow-up idea to the above is that more complex routes may assoc extra information onto the request at the matching stage:
(def echo-first-path-component-route
(GET "/:fst/*" [fst] fst))
This responds with a :body of "foo" to the request from the previous example.
Two things are new about this latest example: the "/:fst/*" and the non-empty binding vector [fst]. The first is the aforementioned Rails-and-Sinatra-like syntax for URI patterns. It's a bit more sophisticated than what is apparent from the example above in that regex constraints on URI segments are supported (e.g. ["/:fst/*" :fst #"[0-9]+"] can be supplied to make the route accept only all-digit values of :fst in the above). The second is a simplified way of matching on the :params entry in the request map, which is itself a map; it's useful for extracting URI segments from the request, query string parameters and form parameters. An example to illustrate the latter point:
(defroutes echo-params
(GET "/" [& more]
(str more)))
user> (echo-params
{:server-port 80
:server-name "127.0.0.1"
:remote-addr "127.0.0.1"
:uri "/"
:query-string "foo=1"
:scheme :http
:headers {}
:request-method :get})
{:status 200,
:headers {"Content-Type" "text/html"},
:body "{\"foo\" \"1\"}"}
This would be a good time to have a look at the example from the question text:
(defroutes main-routes
(GET "/" [] (workbench))
(POST "/save" {form-params :form-params} (str form-params))
(GET "/test" [& more] (str "<pre>" more "</pre>"))
(GET ["/:filename" :filename #".*"] [filename]
(response/file-response filename {:root "./static"}))
(ANY "*" [] "<h1>Page not found.</h1>"))
Let's analyse each route in turn:
(GET "/" [] (workbench)) -- when dealing with a GET request with :uri "/", call the function workbench and render whatever it returns into a response map. (Recall that the return value might be a map, but also a string etc.)
(POST "/save" {form-params :form-params} (str form-params)) -- :form-params is an entry in the request map provided by the wrap-params middleware (recall that it is implicitly included by defroutes). The response will be the standard {:status 200 :headers {"Content-Type" "text/html"} :body ...} with (str form-params) substituted for .... (A slightly unusual POST handler, this...)
(GET "/test" [& more] (str "<pre> more "</pre>")) -- this would e.g. echo back the string representation of the map {"foo" "1"} if the user agent asked for "/test?foo=1".
(GET ["/:filename" :filename #".*"] [filename] ...) -- the :filename #".*" part does nothing at all (since #".*" always matches). It calls the Ring utility function ring.util.response/file-response to produce its response; the {:root "./static"} part tells it where to look for the file.
(ANY "*" [] ...) -- a catch-all route. It is good Compojure practice always to include such a route at the end of a defroutes form to ensure that the handler being defined always returns a valid Ring response map (recall that a route matching failure results in nil).
Why this way?
One purpose of the Ring middleware is to add information to the request map; thus cookie-handling middleware adds a :cookies key to the request, wrap-params adds :query-params and/or :form-params if a query string / form data is present and so on. (Strictly speaking, all the information the middleware functions are adding must be already present in the request map, since that is what they get passed; their job is to transform it to be it more convenient to work with in the handlers they wrap.) Ultimately the "enriched" request is passed to the base handler, which examines the request map with all the nicely preprocessed information added by the middleware and produces a response. (Middleware can do more complex things than that -- like wrapping several "inner" handlers and choosing between them, deciding whether to call the wrapped handler(s) at all etc. That is, however, outside the scope of this answer.)
The base handler, in turn, is usually (in non-trivial cases) a function which tends to need just a handful of items of information about the request. (E.g. ring.util.response/file-response doesn't care about most of the request; it only needs a filename.) Hence the need for a simple way of extracting just the relevant parts of a Ring request. Compojure aims to provide a special-purpose pattern matching engine, as it were, which does just that.