## 2.10 Compilation

### 2.10.1 During program development

During program development, programs are normally loaded using the list abbreviation (`?- [load].`). It is common practice to organise a project as a collection of source files and a load file, a Prolog file containing only use_module/[1,2] or ensure_loaded/1 directives, possibly with a definition of the entry point of the program, the predicate that is normally used to start the program. This file is often called `load.pl`. If the entry point is called go, a typical session starts as:

    % swipl
    <banner>

    1 ?- [load].
    <compilation messages>
    true.

    2 ?- go.
    <program interaction>

When using Windows, the user may open `load.pl` from the Windows explorer, which will cause swipl-win.exe to be started in the directory holding `load.pl`. Prolog loads `load.pl` before entering the top level. If Prolog is started from an interactive shell, one may choose the type `swipl -s load.pl`.

### 2.10.2 For running the result

There are various options if you want to make your program ready for real usage. The best choice depends on whether the program is to be used only on machines holding the SWI-Prolog development system, the size of the program, and the operating system (Unix vs. Windows).

#### 2.10.2.1 Using PrologScript

A Prolog source file can be used directly as a Unix program using the Unix `#!` magic start. The same mechanism is useful for specifying additional parameters for running a Prolog file on Windows. The Unix `#!` magic is allowed because if the first letter of a Prolog file is `#`, the first line is treated as a comment.10The `#`-sign can be the legal start of a normal Prolog clause. In the unlikely case this is required, leave the first line blank or add a header comment. To create a Prolog script, make the first line start like this:

> `#!/path/to/swipl` <options> `-s`

Prolog recognises this starting sequence and causes the interpreter to receive the following argument list:

> `/path/to/swipl` <options> `-s` <script> `--` <ScriptArguments>

Instead of -s, the user may use -f to stop Prolog from looking for a personal initialisation file.

Here is a simple script doing expression evaluation:

    #!/usr/bin/swipl -q -t main -f

    eval :-
            current_prolog_flag(argv, Argv),
            append(_, [--|Args], Argv),
            concat_atom(Args, ' ', SingleArg),
            term_to_atom(Term, SingleArg),
            Val is Term,
            format('~w~n', [Val]).

    main :-
            catch(eval, E, (print_message(error, E), fail)),
            halt.
    main :-
            halt(1).

And here are two example runs:

    % eval 1+2
    3
    % eval foo
    ERROR: Arithmetic: `foo/0' is not a function
    %

The Windows version supports the `#!` construct too, but here it serves a rather different role. The Windows shell already allows the user to start Prolog source files directly through the Windows file-type association. However, Windows makes it rather complicated to provide additional parameters for opening an individual Prolog file. If the file starts with `#!`, the first line is analysed to obtain additional command line arguments. The example below runs the system in `quiet' mode.

    #!/usr/bin/swipl -q -s

    ....

Note the use of `/usr/bin/swipl`, which specifies the interpreter. This argument is ignored in the Windows version, but must be present to ensure best cross-platform compatibility.

#### 2.10.2.2 Creating a shell script

With the introduction of PrologScript (see section 2.10.2.1), using shell scripts as explained in this section has become redundant for most applications.

Especially on Unix systems and not-too-large applications, writing a shell script that simply loads your application and calls the entry point is often a good choice. A skeleton for the script is given below, followed by the Prolog code to obtain the program arguments.

    #!/bin/sh

    base=<absolute-path-to-source>
    PL=swipl

    exec $PL -q -f '$base/load -t go -- **

    go :-
            current_prolog_flag(argv, Arguments),
            append(_SytemArgs, [--|Args], Arguments), !,
            go(Args).

    go(Args) :-
            ...

On Windows systems, similar behaviour can be achieved by creating a shortcut to Prolog, passing the proper options or writing a `.bat` file.

#### 2.10.2.3 Creating a saved state

For larger programs, as well as for programs that are required to run on systems that do not have the SWI-Prolog development system installed, creating a saved state is the best solution. A saved state is created using qsave_program/[1,2] or the -c command line option. A saved state is a file containing machine-independent11The saved state does not depend on the CPU instruction set or endianness. Saved states for 32- and 64-bits are not compatible. Typically, saved states only run on the same version of Prolog on which they have been created. intermediate code in a format dedicated for fast loading. Optionally, the emulator may be integrated in the saved state, creating a single file, but machine-dependent, executable. This process is described in chapter 10.

#### 2.10.2.4 Compilation using the -c command line option

This mechanism loads a series of Prolog source files and then creates a saved state as qsave_program/2 does. The command syntax is:

    % swipl [option ...] [-o output] -c file.pl ...

The options argument are options to qsave_program/2 written in the format below. The option names and their values are described with qsave_program/2.

> `--`option-name`=`option-value

For example, to create a stand-alone executable that starts by executing main/0 and for which the source is loaded through `load.pl`, use the command

    % swipl --goal=main --stand_alone=true -o myprog -c load.pl

This performs exactly the same as executing

    % swipl
    <banner>

    ?- [load].
    ?- qsave_program(myprog,
                     [ goal(main),
                       stand_alone(true)
                     ]).
    ?- halt.