Destructuring Syntax

James Reeves edited this page May 29, 2017 · 29 revisions

Compojure supports two sorts of destructuring for parameters passed in on the request:

  1. The Clojure kind, as one might use in a let or with function arguments.
  2. A Compojure-specific kind designed for parsing parameters from the request map.

Prerequisite Knowledge

This document assumes you:

  1. have read the previous wiki pages.
  2. understand Clojure destructuring.
  3. are familiar with Ring concepts, especially the request map.

Note that many of the examples in this document assume that common Ring middleware has already been applied to the request. Ring-Defaults provides a quick way of applying this middleware.

Regular Clojure Destructuring

This is covered by the standard Clojure Documentation.

Passing one symbol causes the entire request map to be bound to that variable. For example:

(GET "/" request
  (str request))

This example binds the request map to request.

A map can be passed to define a more complex destructuring:

(GET "/user" {{:keys [user-id]} :session}
  (str "The current user is " user-id))

Compojure-specific Destructuring

Since regular destructuring can be quite verbose, Compojure offers a more specialised form of destructuring. If you supply a vector, Compojure will use this custom destructuring syntax. The above example can be written more concisely as:

(GET "/user/:id" [id]
  (str "The user ID is " id))

Notice that the variable name inside the vector ([id]) matches the name of the parameter placeholder (:id).

Query parameters are handled in the same way. Building on our example from an earlier wiki page we can modify our /user/:id route to accept a query string parameter as well.

(GET "/user/:id" [id greeting]
  (str "<h1>" greeting " user " id "</h1>"))

Some cURL examples demonstrating usage of the above route:

$ curl http://localhost:3000/user/jack?greeting=hello
<h1>hello user jack</h1>

$ curl http://localhost:3000/user/jack?greeting=goodbye
<h1>goodbye user jack</h1>

$ curl http://localhost:3000/user/jack
<h1> user jack</h1>

Compojure's parameter destructuring syntax has three pieces of functionality. The first is to bind parameters directly to symbols of the same name. For instance, assume we have the following request map (we will continue to use this request map throughout the rest of this page):

{:request-method :get
 :uri "/foobar"
 :headers {}
 :params {:x "foo", :y "bar", :z "baz", :w "qux"}}

Then we can use a vector of symbols to bind each parameter:

(GET "/foobar" [x y z]
  (str x ", " y ", " z))

Requesting this route will create the response "foo, bar, baz". Notice that the w parameter, while a part of the request map, is not bound to a variable.

To bind a map of all unassigned parameters, you can use the "&" symbol, followed by a variable name:

(GET "/foobar" [x y & z]
  (str x ", " y ", " z))

Testing the above route with cURL:

$ curl "http://localhost:3000/foobar?x=foo&y=bar&z=baz&w=qux"
foo, bar, {:z "baz", :w "qux"}

we can see that the variable x has been assigned the value of "foo", y has been assigned the value of "bar" and z is a map of the remaining parameters.

This behaviour is analogous to the "&" as it is used in normal Clojure bindings. The difference is that instead of getting the unbound remainder as a list, we get the unbound remainder as a map.

Request binding

If you want to access other parts of the request map, you can use the :as keyword to assign the entire request map to a symbol:

(GET "/foobar" [x y :as r]
  (str x ", " y ", " r))

The above route will have the following variable assignments

x = "foo"
y = "bar"
r = {:request-method :get
     :uri            "/foobar"
     :headers        {}
     :params         {:x "foo", :y "bar", :z "baz", :w "qux"}}

You can also bind a Clojure destructuring map to the :as keyword. This will allow specific keys in the request map to get bound to a named variable and the rest of the request map will not be bound (i.e. the rest of the request map will not be available to the code that creates the response). Here's how to use the :as keyword to get only specific keys from the request map:

(GET "/foobar" [x y :as {u :uri rm :request-method}]
  (str "'x' is \"" x "\"\n"
       "'y' is \"" y "\"\n"
       "The request URI was \"" u "\"\n"
       "The request method was \"" rm "\""))

Again, using cURL we can see how the variables get assigned:

$ curl "http://localhost:3000/foobar?x=foo&y=bar&z=baz&w=qux"
'x' is "foo"
'y' is "bar"
The request URI was "/foobar"
The request method was ":get"

Parameter coercion

As of Compojure 1.4.0, you can also supply coercion functions for parameters using the :<< keyword:

[x :<< as-int]

In the above case, the parameter x will be passed through the as-int function before being assigned. If any coercion function returns nil, then the coercion is considered to have failed, and the route will not match.

The compojure.coercions namespace contains two built-in coercions, as-int and as-uuid.

Examples

Regular Clojure Destructuring

  • Get the request map directly:
(GET "/" request
  (str request))

; Output:
; {:server-port 3000, :form-params {}, :compojure/route [:get "/"], ...}
  • Bind a specific part of the request map:
(GET "/user" {{:keys [user-id]} :session}
  (str "The current user ID is " user-id))

; Output for "GET /user" for a logged in user 'jack'
; The current user ID is jack

Compojure Specific Destructuring

Binding a parameter in the route:

(GET "/user/:id" [id]
  (str "The user ID is " id))

; Output for "GET /user/jack"
; The user ID is jack

Binding a route parameter and query parameter:

(GET "/user/:id" [id greeting]
  (str "<h1>" greeting " user " id "</h1>"))

; Output for "GET /user/jack?greeting=farewell"
; <h1>farewell user jack</h2>

Binding three query parameters:

(GET "/foobar" [x y z]
  (str x ", " y ", " z))

; Output for "GET /foobar?x=foo&y=bar&z=baz"
; foo, bar, baz

Mapping unassigned parameters:

(GET "/foobar" [x y & z]
  (str x ", " y ", " z))

; Output for "GET /foobar?x=foo&y=bar&z=baz&w=qux"
; foo, bar, {:z "baz", :w "qux"}

Binding parameters and the entire request map:

(GET "/foobar" [x y :as r]
  (str x ", " y ", " r))

; Output for "GET /foobar?x=foo&y=bar&z=baz"
; foo, bar, {:server-port 3000, ...}

Binding specific keys from the request map:

(GET "/foobar" [x y :as {u :uri rm :request-method}]
  (str "'x' is \"" x "\"\n"
       "'y' is \"" y "\"\n"
       "The request URI was \"" u "\"\n"
       "The request method was \"" rm "\""))

; Output for "GET /foobar?x=foo&y=bar"
; 'x' is "foo" 'y' is "bar" The request URI was "/foobar" The request method was ":get"

Binding a POST form parameter:

(POST "/foo" [name]
  (create-foo name))
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