web service suggestions ?

hi there,

i need to implement web services in my rails 2.0.2 app that external
(non-rails) apps will consume. whilst i'd prefer to do things the
REST-full way, i'm struggling to work out what i need to supply (a la
WSDL) to the other apps so that they know the structure of the request
that they must send me.

i have looked at activeresource, but that appears to be more
appropriate when i've got a rails app to consume web services and wadl
doesn't seem established enough as an alternative to wsdl.

unless i can sort this problem out, i imagine i'll need to go back to
activewebservice. so, am i going about this the wrong way and missing
something blindingly obvious, or is this something that people simply
haven't needed to do in rails ?