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intro - introduction to Limbo modules for the Inferno system


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.

include "module.m";
module := load Module Module->PATH;


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 accessible 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 execution. 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.

The Limbo modules are identical on any machine that is running 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).

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 currently 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:

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.
A module detected the specified internal error. This is most often used for cases where a particular 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:*" =>

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).


draw-intro(2), exception(2), keyring-intro(2), math-intro(2), prefab-intro(2), security-intro(2), sys-intro(2)

INTRO(2 ) Rev:  Tue Mar 31 02:42:39 GMT 2015