Specification of Reactive Hardware/Software Systems - Electronic ...

268 Process Part of POOSL
where PS is a typical element of the set ParStat of parameterised data statements.
PS :: E
¡
¡
¡
x : PE
u : PE
PS1;PS2
if PE then PS1 else PS2 fi
¡
do PE then PS od ¡
¥ L Chan denotes a collection of channels and f is a so-called channel renaming function.
The collection of all channel renaming functions is denoted ChanRen and ranges over
¥¡ ¡ ¡ f .
ChanRen Chan £ Chan
The first two parameterised behaviour specifications C p (PE1 ¥¡ ¡ ¡ ¥ PEr) and C c (PE1 , ¡ ¡
, PEr) denote a single (parameterised) instance of some process class C p y1 ¥¡ ¡ ¡ ¥ yr and
some cluster class C c P1 ¥¡ ¡ ¡ ¥ Pr respectively.
The next kind of specifications BSpec p
1
fications BSpec p
1 and BSpec p
BSpecp2
expresses the parallel composition of speci-
¢
2. Assume, for example, that the class definition of a class, say
Cc ¥ P2 P1 , contains behaviour specification C p
1 (P1) C ¢ p
2 (P2). This specification expresses
the behaviour of two (parameterised) instances, one of some process class C p
y1 1 and
the other of some process class Cp , which execute in parallel and which (perhaps)
y2
communicate through their common channels. The channel set of class Cc ¥ P2 P1 is the
union of the channel sets of classes Cp and C y1 p . An instance of class C y2 c ¥ P2 P1
can send and receive any message which can be sent and received by instances of either
C p
y1 1 or C p
y2 2 . The parallel composition combinator is comparable with the
composition combinator of CCS.
The fourth kind of behaviour specification is called channel hiding. A channel hiding
BSpecp L expresses a specification BSpecp from which the channels in L are made
unobservable. This means that other (external) instances cannot communicate through
channels in L with instances contained in specification BSpecp . Assume, for example, that
the method definition of a class Cc ¥ P2 P1 contains a behaviour specification (C p
¢ 1(P1)
C p
2(P2))
£
. This means that, even though classes C ch¦ p
y1 1 and C p
y2 2 may contain
channel ch, class Cc ¥ P2 P1 may not. Channel ch may only be used for the communication
between (parameterised) instances C p
1 (P1) and C p
2 (P2). The channel hiding constructor is
similar to the restriction combinator of CCS.
The last sort of specification expresses a channel renaming. The channel renaming BSpec p
§ f denotes a specification BSpec p from which the channels are renamed as dictated
by f . We shall often write ch¡1 © ch1 ¥¡ ¡ ¡ ¥ ch¡n © chn for the renaming function f for which
f (chi) ch¡i for i 1¥¡ ¡ ¡ ¥ n and f (ch) ch otherwise. Channel renaming can be very
useful if several instances of the same class are used within different environments and
have to communicate through different channels.