# Designing modules in the Yacas scripting language

##

Introduction

For any software project where the source code grows to a substantial amount of different modules, there needs to be a way to define interfaces between the modules, and a way to make sure the modules don't interact with the environment in an unintended way.

One hallmark of a mature programming language is that it supports modules, and a way to define its interface while hiding the internals of the module. This section describes the mechanisms for doing so in the Yacas scripting language.

##

Demonstration of the problem

Unintentional interactions between two modules typically happen when the two modules accidentally share a common "global" resource, and there should be a mechanism to guarantee that this will not happen.

The following piece of code is a little example that demonstrates the problem:

    SetExpand(fn_IsString) <-- [expand:=fn;];
    ram(x_IsList)_(expand != "") <-- ramlocal(x);
    expand:="";
    ramlocal(x) := Map(expand,{x});

This little bit of code defines a function `ram` that calls the function `Map`, passing the argument passed if it is a string, and if the function to be mapped was set with the `SetExpand` function. It contains the following flaws:

- `expand` is a global variable with a rather generic name, one that another module might decide to use.
- `ramlocal` was intended to be used from within this module only, and doesn't check for correctness of arguments (a small speed up optimization that can be used for routines that get called often). As it is, it can be called from other parts, or even the command line.
- the function `ramlocal` has one parameter, named `x`, which is also generic (and might be used in the expression passed in to the function), and `ramlocal` calls `Map`, which calls `Eval` on the arguments.

The above code can be entered into a file and loaded from the command line at leisure. Now, consider the following command line interaction after loading the file with the above code in it:

    In> ramlocal(a)
    In function "Length" :
    bad argument number 1 (counting from 1)
    Argument matrix[1] evaluated to a
    In function call  Length(a)
    CommandLine(1) : Argument is not a list

We called `ramlocal` here, which should not have been allowed.

    In> ram(a)
    Out> ram(a);

The function `ram` checks that the correct arguments are passed in and that `SetExpand` was called, so it will not evaluate if these requirements are not met.

Here are some lines showing the functionality of this code as it was intended to be used:

    In> SetExpand("Sin")
    Out> "Sin";
    In> ram({1,2,3})
    Out> {Sin(1),Sin(2),Sin(3)};

The following piece of code forces the functionality to break by passing in an expression containing the variable `x`, which is also used as a parameter name to `ramlocal`.

    In> ram({a,b,c})
    Out> {Sin(a),Sin(b),Sin(c)};
    In> ram({x,y,z})
    Out> {{Sin(x),Sin(y),Sin(z)},Sin(y),Sin(z)};

This result is obviously wrong, comparing it to the call above. The following shows that the global variable `expand` is exposed to its environment:

    In> expand
    Out> "Sin";

##

Declaring resources to be local to the module

The solution to the problem is `LocalSymbols`, which changes every symbol with a specified name to a unique name that could never be entered by the user on the command line and guarantees that it can never interact with the rest of the system. The following code snippet is the same as the above, with the correct use of `LocalSymbols`:

    LocalSymbols(x,expand,ramlocal) [
      SetExpand(fn_IsString) <-- [expand:=fn;];
      ram(x_IsList)_(expand != "") <-- ramlocal(x);
      expand:="";
      ramlocal(x) := Map(expand,{x});
    ];

This version of the same code declares the symbols `x`, `expand` and `ramlocal` to be local to this module.

With this the interaction becomes a little bit more predictable:

    In> ramlocal(a)
    Out> ramlocal(a);
    In> ram(a)
    Out> ram(a);
    In> SetExpand("Sin")
    Out> "Sin";
    In> ram({1,2,3})
    Out> {Sin(1),Sin(2),Sin(3)};
    In> ram({a,b,c})
    Out> {Sin(a),Sin(b),Sin(c)};
    In> ram({x,y,z})
    Out> {Sin(x),Sin(y),Sin(z)};
    In> expand
    Out> expand;

##

When to use and when not to use `LocalSymbols`

The `LocalSymbols` should ideally be used for every global variable, for functions that can only be useful within the module and thus should not be used by other parts of the system, and for local variables that run the risk of being passed into functions like `Eval`, `Apply`, `Map`, etc. (functions that re-evaluate expressions).

A rigorous solution to this is to make all parameters to functions and global variables local symbols by default, but this might cause problems when this is not required, or even wanted, behaviour.

The system will never be able to second-guess which function calls can be exposed to the outside world, and which ones should stay local to the system. It also goes against a design rule of Yacas: everything is possible, but not obligatory. This is important at moments when functionality is not wanted, as it can be hard to disable functionality when the system does it automatically.

There are more caveats: if a local variable is made unique with `LocalSymbols`, other routines can not reach it by using the `UnFence` construct. This means that `LocalSymbols` is not always wanted.

Also, the entire expression on which the `LocalSymbols` command works is copied and modified before being evaluated, making loading time a little slower. This is not a big problem, because the speed hit is usually during calculations, not during loading, but it is best to keep this in mind and keep the code passed to `LocalSymbols` concise.