 |
<Core Language> |
[
Parameter Mechanisms |
Garbage Collection |
Prototype Functions |
service/session/seslet |
Miscellaneous
]
All assignments (including implicit assignments of actual parameters
to formal parameters induced by function calls) involving basic types in <bigwig> are call-by-value. Those
involving composite types are
performed by copy-on-write, which is semantically equivalent
to call-by-value, only much more efficient.
All values in <bigwig> are garbage collected when they are
no longer live (reachable by program variables). This happens
incrementally, automatically, and transparently via a built-in
reference counting garbage collector.
<bigwig> is equipped with the possibility of
interacting with functions written (externally) in C. This is
done through prototype functions (i.e. function declarations
with no body). Whenever the <bigwig> compiler encounters such a
declaration, it will assume that the body of the function is provided
by some C code. This can sometimes be useful, when
constructions that are not directly provided by <bigwig> are needed. [See the getting started tutorial].
- service
- A <bigwig> program is also referred to as a Web
service and begins with the keyword service. A service is
comprised of a number of toplevel declarations plus a
number of sessions.
- session
- Sessions are the entry points to the Web service, much like the
main routine in C and Java programs. However, unlike C and Java
programs, a <bigwig> service may have more than one
session. A session contains a number of toplevel
declarations plus a sequence of statements that is executed when the
session is invoked (via a CGI request). This
sequential action may at various points chose to interact with the
client who invoked the session (almost reversing the roles of client
and server), asking for values to be entered and submitted. Unlike CGI
programs, <bigwig> sessions preserve the state across
interactions. A session gives rise to a session
thread (i.e. a process) on the Web server as depicted below:
- seslet
- Seslets provide a more liberal interaction mechanism than that of
the session centered model in which the client and server must
strictly alternate between being active and suspended. There are two
kinds of seslets; client-side and server-side seslets. A client-side
seslet is a Java applet that can run in the browser and request
information from the server-side seslets. For each service, the
<bigwig> compiler generates the Java code for
a superclass, itself a subclass of Applet. The programmer can
then extend this Java class and use it to access the available
server-side seslets. Server-side seslets are functions that can be
invoked by client-side seslets in order to compute information or
produce side-effects. Since Server-side seslets do not have a client,
they are not allowed to show or exit documents.
If the client submits the current page, then the browser terminates
all running client-side seslet applets. However, running server-side
seslets are allowed to run until termination. This is necessary for
instance to avoid breaking critical synchronization invariants.
- getenv
- This built-in function takes a string argument naming an environment
variable and returns the (string) value of this variable.
- random
- This built-in function takes an int value and returns a
(pseudo-) random int value between (including both end
points) zero and the value given minus one.
- system
- This built-in function is similar to C's system function. It takes
a string which is executed in a shell (sh) and returns the (integer) exit
status produced by the shell. The path is the $SERVICEDIR/<<service-name>>.
Use with caution!
- Expression-Statements
- Statements can now be embedded in expressions (exp ::=
'(' compound_stm ')') as in Gnu C. The
result is the value of the last statement if it is a
statement-expression (stm ::= exp ';') and void
otherwise.
- typeof
- This construct takes an expression in parentheses. The type of the
construct is equal to the type of the expression. Note that the
expression is solely used for determining the type and is thus never
run. The construction is mostly intended for use via the syntax
macros.
