INTRO(2) INTRO(2)
NAME
intro - introduction to Limbo modules for the Inferno system
SYNOPSIS
include "sys.m";
sys := load Sys Sys->PATH;
include "draw.m";
draw := load Draw Draw->PATH;
include "tk.m";
tk := load Tk Tk->PATH;
... etc.
Generically:
include "module.m";
module := load Module Module->PATH;
DESCRIPTION
This section introduces the Limbo modules available to the
programmer; see the corresponding manual pages for more
information. Each module is declared with a single Limbo
include file. Before calling a module's functions, an
application must load the module; the application stores the
resulting value in a variable for later use as the module
qualifier. The examples above illustrate the style. It
will usually be necessary in some cases to qualify names
with the appropriate module pointer or to import the types
and functions; the manual pages assume the names are acces-
sible in the current scope.
Although many modules are self-contained, dependencies may
exist. For example, the system module, Sys, provides basic
services that many other modules require. These are the
Inferno equivalent to `system calls'.
In a few cases, several related modules share a single
include file; for instance, security.m.
The manual pages describe how to include a module definition
during compilation and load an implementation during execu-
tion. The documentation also lists relevant functions or
abstract data types. Although the include files declare
these components, the manual pages list them explicitly. In
all cases, the enclosing module declaration is assumed so
that unqualified identifiers can be used in the text without
ambiguity, reducing clutter in the text. In practice when
programming, many consider it good style to use an explicit
module reference for functions and constants.
Page 1 Plan 9 (printed 12/22/24)
INTRO(2) INTRO(2)
The Limbo modules are identical on any machine that is run-
ning Inferno, whether native or hosted, which enables Limbo
programs to be written and tested on any Inferno system.
Many modules are described in a single page, such as
regex(2). Several larger modules are explained in several
sections, such as math-intro(2), math-elem(2), math-fp(2),
and math-linalg(2).
Exceptions
Exception handling is now part of the Limbo language,
replacing an older scheme that used special system calls.
Various exceptions can be raised by the virtual machine when
run-time errors are detected. These are the common ones:
alt send/recv on same chan
It is currently illegal for a channel to appear in
two alt statements if they either both receive or
both send on it. (It is fine to send in one and
receive in the other.)
array bounds error
Array subscript out of bounds.
dereference of nil
Attempt to use a ref adt or index an array with
value nil .
invalid math argument
Inconsistent values provided to functions of
math-intro(2).
module not loaded
Attempt to use an uninitialised module variable.
negative array size
The limit in an array constructor was negative.
out of memory: pool
The given memory pool is exhausted. Pool is cur-
rently one of main (kernel memory including Tk
allocations), heap (most Limbo data), and image
memory for draw(3).
zero divide
Integer division (or mod) by zero.
There are currently two more classes of exception string
with a conventional interpretation imposed not by the run-
time system proper, but by Limbo components:
fail:reason
Page 2 Plan 9 (printed 12/22/24)
INTRO(2) INTRO(2)
Commands use this exception to provide an `exit
status' to a calling program, particularly the
shell sh(1); see also sh(2). The status is given
by the reason following the `fail:' prefix.
assertion:error
A module detected the specified internal error.
This is most often used for cases where a particu-
lar possibility ``cannot happen'' and there is no
other need for an error value in the interface.
Otherwise, most module interfaces tend to use explicit error
return values, not exceptions.
Note that a Limbo exception handler can do pattern matching
to catch a class of exceptions:
{
body of code to protect
} exception e {
"out of memory:*" =>
recovery action
"assertion:*" =>
fatal_error(e);
}
The effect of an unhandled exception in a process that is
part of an error-recovery group can be controlled using the
mechanisms described in prog(3) as accessed using
exception(2).
SEE ALSO
draw-intro(2), exception(2), keyring-intro(2), math-
intro(2), prefab-intro(2), security-intro(2), sys-intro(2)
Page 3 Plan 9 (printed 12/22/24)