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::: 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.