Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!bloom-beacon!ATHENA.MIT.EDU!swick
From: swick@ATHENA.MIT.EDU (Ralph R. Swick)
Newsgroups: comp.windows.x
Subject: Re: A Call for Callforwards...
Message-ID: <8906291555.AA02112@LYRE.MIT.EDU>
Date: 29 Jun 89 15:55:56 GMT
References: <8906260010.AA16100@bu-cs.BU.EDU>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: DEC/MIT Project Athena
Lines: 17

> A Callforward is simply an application-supplied function which the
> widget calls (if it exists) and expects a return value from.
> ...
> The definition is almost identical to that of a Callback but the
> return type is changed from void to some generic type (eg. caddr_t).

I certainly agree with you that widgets could be calling into
application code to retrieve information, not just to provide it,
and that this would make certain APIs less clumsy.

But we don't need to add a new (procedure) datatype to support this
in Xt; the call_data argument in XtCallbackProc is defined by the
widget to be whatever it needs, including a pointer to a return
value from the callback.

The syntax you propose for callforwards has a syntactic advantage for
single-valued procedures but doesn't actually extend the bag o' tricks.