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context-guarded prefix
and it has the form
?.
. Such a process waits until the environment
satisfies the context expression
κ
, then performs the capability
M
and continues like the
process
P
. The process learns about its context (i.e. its environment) by evaluating the guard.
The use of context-guarded capabilities is one of the two main mechanisms for context
acquisition in CCA (the second mechanism for context acquisition is the call to a process
abstraction as discussed below). The syntax and the semantics of context expressions are
given below. We let
M.P
denote the process
True
?
M.P
, where
True
is a context expression
satisfied by all context.
,, ∷=
Process
0
inactivity
P|Q
parallel composition
(
υn
)
P
name restriction
n
[
P
]
ambient
!P
repliaction
κ!M.P
context-guarder action
x⊳(y).P
process abstraction
∷=
Locations
↑
any parent
n↑
parent n
↓
any child
n↓
child n
∷
any sibling
n∷
sibling n
ϵ
locally
∷=
Capabilities
del
n
delete n
in
n
move in n
out
move out
αx
〈
z
〉
process call
α
(
y
)
input
α〈y〉
output
Table 1. Syntax of CCA processes and capabilities
A process abstraction
⊳
(
)
.
denotes the linking of the name
x
to the process
P
where
is
a list of
formal parameters
. This linking is local to the ambient where the process abstraction is
defined. So a name
x
can be linked to a process
P
in one ambient and to a diferent process
Q
in another ambient. A call to a process abstraction named
x
is done by a capability of the
form
〈
̃
〉
where
specifies the location where the process abstraction is defined and
̃
is
the list of
actual parameters
. There must be as many actual parameters as there are formal
parameters to the process abstraction being called. The location
can be
′↑′
for any parent,
′↑′
for a specifc parent
n
,
′↓′
for any child,
′↓′
for a specific child
n
,
′∷′
for any sibling,
′∷
for a specific sibling
n
, or
(empty string) for the calling ambient itself. A process call
