- use Sub::Name to set anonymous functions names
# a few shots at getting a usable syntax for
# a source filter variant of the contract declaration
#-------------------------------------
#
# simple way
#
#-------------------------------------
package MyPkg;
use Sub::Contract;
contract('foo',
               in => [ &valid1, &valid2, a => &valid3, b => &valid4 ],
               out => [ &valid1, a => &valid3 ],
               cache => SIZE,
);
# contract_in/out dies if foo already declared previously
sub foo {
    return @aa;
}
# since we have encountered a contract declaration
# for both in and out arguments for foo, 
# source filter replaces foo by:
sub foo {
    Sub::Contract::validate_MyPkg_foo_in(@_);
    return Sub::Contract::validate_MyPkg_foo_out @aa;
}
# in Sub::Contract:
sub UNIVERSAL {
    # called when Sub::Contract::validate_... called from somewhere
    # if ... validator does not exist, create it!
}
# EVAL: 
# - quite efficient: call validate_... only when a contract exists.
# - easy to enable/disable contracts during runtime by putting
#    a 'return if disabled' in beginning of validate_...
# - problem: no support for variable lenght argument lists in contract_in/out
# - a bit hard to parse all perl variants of sub { and return declarations (but
#    not too hard)
#-------------------------------------
#
# the function prototype way
#
#-------------------------------------
sub foo :contract(in => {}, out => {})