Manual

Hello World

After installing Exodus, you can develop Exodus programs from any operating system command line.

This introduction concentrates on simple development from the command line because it is common to all platforms.

Edit

Use Exodus's all-in-one editor+compiler+cataloger 'edic'

edic hello1

edic will give you a skeleton Exodus program which you can develop as you wish.

There must be one and only one "function main()" statement and this is run when the program is started.

By convention, main() must return an integer value. If desired, this can be used to indicate "success" with zero or "failure" with some error number.

The programinit() and programexit() lines are required and provide all the standard multivalue system variables using a simple C++ class macro.

#include <exodus/program.h>

programinit()

function main() {
        printl("hello1 says 'Hello World!'");
        return 0;
}

programexit()

Save and compile

Make any changes you fancy to the skeleton and save it.

NB If you just want to try out the skeleton program exactly as it is then you must explicitly save it - otherwise if you just exit without saving, edic will assume you have changed your mind and that no longer want the hello1 program, and will cancel. For the default editor (nano), explicit save is usually Ctrl+O.

On saving hello1, edic will compile and catalog it.

Run

To run/open/execute your new hello1 program just type its name.

hello1

and the output is ...

hello1 says 'Hello World!'

Deployment of Compiled Programs

All programs you develop with Exodus's edic or compile commands are stored in the same directory as their source code. They are also copied to a folder in your home directory. This folder is called "Exodus" on Windows and bin or lib on other operating systems.

On Windows, the installing user can run any program they develop in Exodus by typing its name at any command prompt, regardless of the current working directory because the Exodus installer permanently adds the Exodus home program directory to the PATH of the installing user.

For Linux, Mac and other operating systems, before you can run any program you develop, you must run "exodus" from a command prompt or application menu. Thereafter you can run any program you have developed in Exodus by typing its name at any command prompt, regardless of the current working directory because the "exodus" program temporarily adds the Exodus home program directory to your PATH.

On all operating systems, if there is a similarly named program earlier on in your path (e.g. operating system commands) then you will not be able to run your program because Exodus adds its paths to the end of your existing path, not the beginning.

To make the Exodus programs that you develop available to other users you must arrange for them to be copied to some directory on their path.

Local subroutines

To simulate classic multivalue basic's "gosub/return" in Exodus, you can add additional subroutines and functions above or below your "main" function.

Local subroutines and functions (including the "main" function) can be any order. There is no rule that functions must appear before or above the code that calls them.

All subroutines and functions must be placed between the programinit() and programexit() lines.

Subroutines and functions cannot be nested otherwise compilation will fail.

Simple Example

Here is hello1 modified to call a subroutine that says something.

The word "gosub" is just there for classical clarity. It could be omitted. It could also be replaced by "call" which is also a throwaway word. While gosub and call are interchangeable, if you are going to use them at all, it is probably a good idea to use "gosub funcx()" to indicate internal subroutines and "call funcx()" to indicate external subroutines.

#include <exodus/program.h>

programinit()

function main() {
        printl("hello1 says 'Hello World!'");

        gosub subr1();

        return 0;
}

subroutine subr1() {
        printl("subr1 says 'Hello'");
}

programexit()

output:

hello1 says 'Hello World!'
subr1 says 'Hello'

Subroutine and Function Parameters (Arguments)

Parameter names must be prefixed by "in", "out" or "io" to indicate if they are inbound, outbound or both.

Inbound parameters (prefixed with "in") cannot be amended within the function and the compiler will enforce this. This provides a guarantee to the programmer calling the subroutine or functions that their inbound parameters will not be modified by calling the function. Inbound parameters may be provided by the calling program either as a variable or as raw data like "xyz" or 123 etc.

In/Outbound parameters ("io") must be provided by the calling program in the form of a variable and not raw data. The function can both read and write into these parameters.

Outbound parameters ("out") indicate to the programmer using the function how it works. However, outbound parameters do not actually prevent them being used to provide data to the subroutine or function and are therefore no different from in/outbound parameters. There is no simple way to enforce "write only" parameters using the underlying C++ language.

For special cases, or where performance is critical, the usual C++ parameters are also allowed for example "int arg1" or "const char* arg2" etc.

External functions and subroutines

Editing and compiling external subroutines and functions in Exodus is identical to normal programs except:

1. the word "program" becomes "library" so we have "library.h", "libraryinit()" and "libraryexit()".
2. function main can have any parameters you like, for example: "function main(in arg1, in arg2, out arg3)

NB The name of the function internally must be main and NOT the name of the function so we still have "function main(...)" but this time it can have parameter names.

External functions and subroutines are only loaded into memory the first time that they are used. A missing function or subroutine will not cause any failure unless and until it is first used.

Example func1

edic func1

#include <exodus/library.h>

libraryinit()

function main(in arg1, out arg2) {
        printl("func1 says 'Hello World!'");
        arg2="arg1 was "^arg1;
        return 999;
}

libraryexit()

Example prog1 using func1

Compiling func1 created a func1.h file that is required to "glue" func1 into any program that wants to use func1.

To use func1 in any program or external subroutine or function, you need to insert an #include "func1.h" somewhere between "programinit()" and "programexit()" - and not within the body of any function or subroutine.

edic prog1

#include <exodus/program.h>

programinit()

//you must "declare" that we want to use func1
#include "func1.h"

function main() {
        printl("prog1 says 'Hello World!'");

        //we print what func1 returns, and also what func1 returns in arg2
        var arg2;
        printl(   func1("myarg1",arg2)   );
        printl(arg2);

        return 0;
}

programexit()

run prog1

prog1

output:

prog1 says 'Hello World!'
func1 says 'Hello World!'
999
arg1 was myarg1

Functions versus Subroutines

Functions can be used anywhere an expression would be used whereas subroutines can only be used as statements.

The same principle applies to both internal and external subroutines/functions.

Both subroutines and functions can return information in any of their "out" or "io" arguments.

The word "call" or "gosub" before the use of a function or subroutine is optional and can be omitted.

Functions

Functions return a value so the calling program can use a function anywhere an expression is required, for example on the right hand side of an assignment.

var yy=funcx(xx);

Even though a function returns a value, it can be ignored.

call funcx(xx);
gosub funcx(xx);
funcx(xx);

Subroutines

Subroutines do not return a value so they can only be used as statements.

call subrx(xx);   
gosub subrx(xx);  
subrx(xx);        
var yy=subrx(xx); //will not compile

Differences between Exodus's Internal Subroutines and Functions and those of classic multivalue basic

Exodus's internal subroutines and functions are very different from and arguably much better than classic multivalue internal subroutines. They are very similar to external subroutines and functions except they also have access to the global variables of the main program.

Exodus Internal Subroutines and Functions:

1. can be called with parameters e.g. gosub funcx(xx,yy,zz)
2. all variables are private (preventing many bugs)
3. have no access to the main programs variables except for program global variables
4. functions can return a result e.g. abc=funcx(xx,yy,zz)

They only have one entry point whereas in classic multivalue basic you can jump into the middle of any internal subroutine. To simulate this type of coding in in Exodus you must create nested subroutines or functions.

The old RETURN TO XYZ syntax is not supported at all and such code must be redesigned to eliminate it.

Moving external subroutines and functions to internal

Exodus allows many classic multivalue basic external subroutines and functions to be reimplemented as internal subroutines and functions. This means that you can now, where appropriate, keep all source code related to one program in a single file. This is a major improvement in source code management.

In classic multivalue basic, if you wanted to implement some function to be used in expression you had to implement it as an external function requiring a separate source code file. If this function was applicable only to one program then splitting it into a separate source file is likely to be an unwanted but unavoidable outcome.

In classic multivalue basic, or you wanted to call a subroutine with parameters, or you wanted to be sure that a subroutine would not accidentally overwrite main program variables, you had to implement it as an external subroutine or function.

You can still make subroutines and functions external where that is the best strategy, for example:

1. to provide some common feature to more than one program
2. to compile a large program in parts
3. where there is some benefit in not loading all the parts of a program into memory immediately on start-up.

NB Exodus internal subroutines and functions have access to the global variables of the main program so you might want them to make them external to make sure that there is no risk that they touch any global variable.