SDL-News: Signal objects as variables

Subject: SDL-News: Signal objects as variables
From: Oystein Haugen (
Date: Mon Oct 06 1997 - 14:42:43 GMT

The originator of this message is responsible for its content.
-----From Oystein Haugen <> to sdlnews -----

Rick Reed (and SDL friends)

Here is a contribution for the Lutterworth expert meeting concerning
generalization of signals.
The pure text is copied below. The full contribution includes also an
example diagram to illustrate these simple ideas. The illustration is
not distributed on this mailing list.

Oystein Haugen

Signal objects as variables

As any SDL implementation has discovered, there is definitely some gain
in harmonizing signals and variables. We suggest the following scheme:
1. Signal data type. We define a new predefined data type which is the
set of all signals. It is designated SIGNAL, such that a declaration of
a signal variable will look like: “dcl my_sig SIGNAL;”. The signal data
type may then also appear as parameter to another signal. It is also
possible to define variables of specific signal types by extending the
syntax: “dcl my_sig SIGNAL mysignaltype;”
2. Output. It is possible to output a stored signal by outputting the
variable: “output my_sig;” The SENDER attribute of the signal is
modified to SELF of this process.
3. Dash signal. We define a predefined function (called DASHSIGNAL)
which returns a SIGNAL value which is equal to the most recently
consumed signal of the process.
4. Check signal type. We define a Boolean function which can be used to
check the signal type of a signal variable: “my_sig is return;”. A
similar construct can then be used to interpret parameters: “my_sig qua
5. Atleast input. We may consume signals of different signal types in
one transition by specifying a supertype of the signal types by “input
atleast return;”. Combination with DASHSIGNAL and signal type check
makes it possible to use this effectively. In general we can use
“atleast signaltype” also in signallists to indicate that any signal
type which is inherited from signal type is allowed.

All together these features make it possible to generalize SDL
communication more wrt. signals.

Oystein Haugen, Ericsson as. , P.O. box 34, N-1361 Billingstad, Norway
Tel: +47 66 84 23 46 Fax: +47 66 98 28 20 E-mail:

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