cowboy/doc/src/guide/constraints.ezdoc

::: Constraints

Cowboy provides an optional constraints based validation feature
when interacting with user input.

Constraints are first used during routing. The router uses
constraints to more accurately match bound values, allowing
to create routes where a segment is an integer for example,
and rejecting the others.

Constraints are also used when performing a match operation
on input data, like the query string or cookies. There, a
default value can also be provided for optional values.

Finally, constraints can be used to not only validate input,
but also convert said input into proper Erlang terms, all in
one step.

:: Structure

Constraints are provided as a list of fields and for each
field a list of constraints for that field can be provided.

Fields are either the name of the field; the name and
one or more constraints; or the name, one or more constraints
and a default value.

When no default value is provided then the field is required.
Otherwise the default value is used.

All constraints for a field will be used to match its value
in the order they are given. If the value is modified by a
constraint, the next constraint receives the updated value.

:: Built-in constraints

||	Constraint	Description
|
|	int			Convert binary value to integer
|	nonempty	Ensures the binary value is non-empty

:: Custom constraint

In addition to the predefined constraints, Cowboy will accept
a fun. This fun must accept one argument and return one of
`true`, `{true, NewValue}` or `false`. The result indicates
whether the value matches the constraint, and if it does it
can optionally be modified. This allows converting the value
to a more appropriate Erlang term.

Note that constraint functions SHOULD be pure and MUST NOT crash.
Breaking update of the cowboy_req interface Simplify the interface for most cowboy_req functions. They all return a single value except the four body reading functions. The reply functions now only return a Req value. Access functions do not return a Req anymore. Functions that used to cache results do not have a cache anymore. The interface for accessing query string and cookies has therefore been changed. There are now three query string functions: qs/1 provides access to the raw query string value; parse_qs/1 returns the query string as a list of key/values; match_qs/2 returns a map containing the values requested in the second argument, after applying constraints and default value. Similarly, there are two cookie functions: parse_cookies/1 and match_cookies/2. More match functions will be added in future commits. None of the functions return an error tuple anymore. It either works or crashes. Cowboy will attempt to provide an appropriate status code in the response of crashed handlers. As a result, the content decode function has its return value changed to a simple binary, and the body reading functions only return on success. 2014-09-23 16:43:29 +03:00			`::: Constraints`

			`Cowboy provides an optional constraints based validation feature`
			`when interacting with user input.`

			`Constraints are first used during routing. The router uses`
			`constraints to more accurately match bound values, allowing`
			`to create routes where a segment is an integer for example,`
			`and rejecting the others.`

			`Constraints are also used when performing a match operation`
			`on input data, like the query string or cookies. There, a`
			`default value can also be provided for optional values.`

			`Finally, constraints can be used to not only validate input,`
			`but also convert said input into proper Erlang terms, all in`
			`one step.`

			`:: Structure`

			`Constraints are provided as a list of fields and for each`
			`field a list of constraints for that field can be provided.`

			`Fields are either the name of the field; the name and`
			`one or more constraints; or the name, one or more constraints`
			`and a default value.`

			`When no default value is provided then the field is required.`
			`Otherwise the default value is used.`

			`All constraints for a field will be used to match its value`
			`in the order they are given. If the value is modified by a`
			`constraint, the next constraint receives the updated value.`

			`:: Built-in constraints`

			`\|\| Constraint Description`
			`\|`
			`\| int Convert binary value to integer`
			`\| nonempty Ensures the binary value is non-empty`

			`:: Custom constraint`

			`In addition to the predefined constraints, Cowboy will accept`
			`a fun. This fun must accept one argument and return one of`
			`true`, `{true, NewValue}` or `false`. The result indicates
			`whether the value matches the constraint, and if it does it`
			`can optionally be modified. This allows converting the value`
			`to a more appropriate Erlang term.`

			`Note that constraint functions SHOULD be pure and MUST NOT crash.`