Field accessors

I'd like to be able to access a field of a record in a variable. For example:

weapon({ name='dagger', die='1d4' }).
weapon({ name='shortsword', die='1d6' }).
d4weapons($x) := weapon($x), @equals($x.die, '1d4').
d4weapons($x)?

d4weapons({ name='dagger', die='1d4' })