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-Routing
-=======
-
-Cowboy does nothing by default.
-
-To make Cowboy useful, you need to map URLs to Erlang modules that will
-handle the requests. This is called routing.
-
-When Cowboy receives a request, it tries to match the requested host and
-path to the resources given in the dispatch rules. If it matches, then
-the associated Erlang code will be executed.
-
-Routing rules are given per host. Cowboy will first match on the host,
-and then try to find a matching path.
-
-Routes need to be compiled before they can be used by Cowboy.
-
-Structure
----------
-
-The general structure for the routes is defined as follow.
-
-``` erlang
-Routes = [Host1, Host2, ... HostN].
-```
-
-Each host contains matching rules for the host along with optional
-constraints, and a list of routes for the path component.
-
-``` erlang
-Host1 = {HostMatch, PathsList}.
-Host2 = {HostMatch, Constraints, PathsList}.
-```
-
-The list of routes for the path component is defined similar to the
-list of hosts.
-
-``` erlang
-PathsList = [Path1, Path2, ... PathN].
-```
-
-Finally, each path contains matching rules for the path along with
-optional constraints, and gives us the handler module to be used
-along with options that will be given to it on initialization.
-
-``` erlang
-Path1 = {PathMatch, Handler, Opts}.
-Path2 = {PathMatch, Constraints, Handler, Opts}.
-```
-
-Continue reading to learn more about the match syntax and the optional
-constraints.
-
-Match syntax
-------------
-
-The match syntax is used to associate host names and paths with their
-respective handlers.
-
-The match syntax is the same for host and path with a few subtleties.
-Indeed, the segments separator is different, and the host is matched
-starting from the last segment going to the first. All examples will
-feature both host and path match rules and explain the differences
-when encountered.
-
-Excluding special values that we will explain at the end of this section,
-the simplest match value is a host or a path. It can be given as either
-a `string()` or a `binary()`.
-
-``` erlang
-PathMatch1 = "/".
-PathMatch2 = "/path/to/resource".
-
-HostMatch1 = "cowboy.example.org".
-```
-
-As you can see, all paths defined this way must start with a slash
-character. Note that these two paths are identical as far as routing
-is concerned.
-
-``` erlang
-PathMatch2 = "/path/to/resource".
-PathMatch3 = "/path/to/resource/".
-```
-
-Hosts with and without a trailing dot are equivalent for routing.
-Similarly, hosts with and without a leading dot are also equivalent.
-
-``` erlang
-HostMatch1 = "cowboy.example.org".
-HostMatch2 = "cowboy.example.org.".
-HostMatch3 = ".cowboy.example.org".
-```
-
-It is possible to extract segments of the host and path and to store
-the values in the `Req` object for later use. We call these kind of
-values bindings.
-
-The syntax for bindings is very simple. A segment that begins with
-the `:` character means that what follows until the end of the segment
-is the name of the binding in which the segment value will be stored.
-
-``` erlang
-PathMatch = "/hats/:name/prices".
-HostMatch = ":subdomain.example.org".
-```
-
-If these two end up matching when routing, you will end up with two
-bindings defined, `subdomain` and `name`, each containing the
-segment value where they were defined. For example, the URL
-`http://test.example.org/hats/wild_cowboy_legendary/prices` will
-result in having the value `test` bound to the name `subdomain`
-and the value `wild_cowboy_legendary` bound to the name `name`.
-They can later be retrieved using `cowboy_req:binding/{2,3}`. The
-binding name must be given as an atom.
-
-There is a special binding name you can use to mimic the underscore
-variable in Erlang. Any match against the `_` binding will succeed
-but the data will be discarded. This is especially useful for
-matching against many domain names in one go.
-
-``` erlang
-HostMatch = "ninenines.:_".
-```
-
-Similarly, it is possible to have optional segments. Anything
-between brackets is optional.
-
-``` erlang
-PathMatch = "/hats/[page/:number]".
-HostMatch = "[www.]ninenines.eu".
-```
-
-You can also have imbricated optional segments.
-
-``` erlang
-PathMatch = "/hats/[page/[:number]]".
-```
-
-You can retrieve the rest of the host or path using `[...]`.
-In the case of hosts it will match anything before, in the case
-of paths anything after the previously matched segments. It is
-a special case of optional segments, in that it can have
-zero, one or many segments. You can then find the segments using
-`cowboy_req:host_info/1` and `cowboy_req:path_info/1` respectively.
-They will be represented as a list of segments.
-
-``` erlang
-PathMatch = "/hats/[...]".
-HostMatch = "[...]ninenines.eu".
-```
-
-If a binding appears twice in the routing rules, then the match
-will succeed only if they share the same value. This copies the
-Erlang pattern matching behavior.
-
-``` erlang
-PathMatch = "/hats/:name/:name".
-```
-
-This is also true when an optional segment is present. In this
-case the two values must be identical only if the segment is
-available.
-
-``` erlang
-PathMatch = "/hats/:name/[:name]".
-```
-
-If a binding is defined in both the host and path, then they must
-also share the same value.
-
-``` erlang
-PathMatch = "/:user/[...]".
-HostMatch = ":user.github.com".
-```
-
-Finally, there are two special match values that can be used. The
-first is the atom `'_'` which will match any host or path.
-
-``` erlang
-PathMatch = '_'.
-HostMatch = '_'.
-```
-
-The second is the special host match `"*"` which will match the
-wildcard path, generally used alongside the `OPTIONS` method.
-
-``` erlang
-HostMatch = "*".
-```
-
-Constraints
------------
-
-After the matching has completed, the resulting bindings can be tested
-against a set of constraints. Constraints are only tested when the
-binding is defined. They run in the order you defined them. The match
-will succeed only if they all succeed.
-
-They are always given as a two or three elements tuple, where the first
-element is the name of the binding, the second element is the constraint's
-name, and the optional third element is the constraint's arguments.
-
-The following constraints are currently defined:
-
- * {Name, int}
- * {Name, function, fun ((Value) -> true | {true, NewValue} | false)}
-
-The `int` constraint will check if the binding is a binary string
-representing an integer, and if it is, will convert the value to integer.
-
-The `function` constraint will pass the binding value to a user specified
-function that receives the binary value as its only argument and must
-return whether it fulfills the constraint, optionally modifying the value.
-The value thus returned can be of any type.
-
-Note that constraint functions SHOULD be pure and MUST NOT crash.
-
-Compilation
------------
-
-The structure defined in this chapter needs to be compiled before it is
-passed to Cowboy. This allows Cowboy to efficiently lookup the correct
-handler to run instead of having to parse the routes repeatedly.
-
-This can be done with a simple call to `cowboy_router:compile/1`.
-
-``` erlang
-Dispatch = cowboy_router:compile([
- %% {HostMatch, list({PathMatch, Handler, Opts})}
- {'_', [{'_', my_handler, []}]}
-]),
-%% Name, NbAcceptors, TransOpts, ProtoOpts
-cowboy:start_http(my_http_listener, 100,
- [{port, 8080}],
- [{env, [{dispatch, Dispatch}]}]
-).
-```
-
-Note that this function will return `{error, badarg}` if the structure
-given is incorrect.
-
-Live update
------------
-
-You can use the `cowboy:set_env/3` function for updating the dispatch
-list used by routing. This will apply to all new connections accepted
-by the listener.
-
-``` erlang
-cowboy:set_env(my_http_listener, dispatch,
- cowboy_router:compile(Dispatch)).
-```
-
-Note that you need to compile the routes before updating.