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Programming in SLI


A procedure is a sequence of SLI objects whose execution is delayed until the procedure is executed. Because procedures are objects, they can be:

  • placed on the stack

  • bound to a name

  • executed repeatedly

  • executed by another procedure

A program is a sequence of SLI objects and procedures which are defined in a file. This section introduces the fundamentals of SLI programming.

This chapter covers the basic programming concepts of the SLI language features.

Entering and executing programs

A program is a sequence of SLI objects and procedures which are defined in a file. Program files are ordinary ASCII text files, which can be created and modified with an editor of your choice (e.g. GNU Emacs).

SLI programs usually have the file ending “sli”, for example hello_world.sli.

The run command is used to execute a program file.

Example: “Hello World!”

Write the program hello_world.sli according to the example, given above.

  1. Create an empty file hello_world.sli in the directory from which the interpreter was started.

  2. Copy the Hello World example to the file and save it.

  3. Enter (hello_world.sli) run at the command prompt.

    /HelloWorld { (Hello World !) = } def

    SLI ] (hello_world.sli) run SLI ] HelloWorld Hello World !

Note that the procedure is not immediately executed by run. Rather, all objects which are contained in the file are read and executed.

Using local variables

Usually, all names you define are globally accessible. But, if you use a lot of procedures that define their own variables, there is an increasing danger that two procedures use the same name for different purposes. This problem can be solved by keeping variable local to the procedure that defines them.

SLI uses dictionaries to store and resolve variables.

Example 2

Compute the alpha-function of t according to a(t)=t*Exp(-t/tau)

  << /tau -1.0 >> % create dictionary for local variables
  begin           % open local name space
    /t exch def     % store argument in local variable t
    t tau div exp   % compute formula
    t mul
  end             % close local name space
} def


Conditional expressions allow a program to ask questions and make decisions:

  • Comparisons and logical functions

  • Conditional structures which test a certain condition and use the result to make a decision.

In general, conditional structures take a boolean object as well as one or more procedure objects as argument and evaluate one of the procedures, depending on the value of the boolean.

Example 3

The program in this example implements the faculty function according to the definition:

fac(1) := 1
fac(n) := n*fac(n-1), for n>1

The program expects the argument on the stack and replaces it by the result. Here, we use the if command to test whether the argument is greater than 1. The if command takes two arguments, a boolean and a procedure object. The boolean is supplied by the gt command which test if the object at stack level 1 is greater than the object at level 0.

  dup    % duplicate the argument, since we still need it.
  1 gt   % If n > 1 we call fac recursively
  {      % according to fac(n)=n*fac(n-1)
    1 sub fac % call fac with n-1 as argument
    mul       % multiply the result with the argument
  } if
} def

This example also shows how procedures can be called recursively. It is, however, important to supply a termination condition for the recursion like in this example.

Comparison functions

Comparison functions are used to compare objects. The result of comparison functions are of type /booltype and can be used for logical functions and conditional structures.

Command Description eq Test whether two objects are equal. ne Test whether two objects are not equal. gt Test whether the object at level 1 is greater than the object at level 0. lt Test whether the object at level 1 is less than the object at level 0. leq Test whether the object at level 1 is less than or equal to the object at level 0. geq Test whether the object at level 1 is greater than or equal to the object at level 0.

Logical functions

Command Description not Negates a bool. and Returns true if both arguments are true. or Returns true if at least one of the arguments is true. xor Returns true if and only if one of the arguments is true.

The if-ifelse structure

Command Description bool proc if Executes proc if the boolean is true. bool proc_1 proc_2 ifelse Executes proc_1 if the boolean is true and proc_2 otherwise.


SLI ] 1 2 eq {(Equal!) = } { (Not equal !) =} ifelse
Not equal !
SLI ] 2 2 eq {(Equal!) = } { (Not equal !) =} ifelse

The case-switch structure

While the commands if and ifelse test only one condition, the case-switch structure can be used to test a number of different conditions.

The case-switch structure has the general form:

 bool_1 proc_1 case
 bool_2 proc_2 case
 bool_n proc_n case

In this structure, proc_i is executed, if the corresponding value of bool_i is true.

Sometimes it is necessary to provide a default procedure, which is evaluated if none of the boolean is true.

The case-switchdefault structure has the general form

 bool_1 proc_1 case
 bool_2 proc_2 case
 bool_n proc_n case

Here, procdefault is executed if none of the booleans was true.


Loops and control structures are commands that take procedure objects as arguments.

Infinite loops

The simplest loop is performed by the command loop:

SLI ] {(Hello World) =} loop
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World

loop performs the procedure repeatedly and thus in the example, an infinite succession of the words “Hello World” is printed. The only way to leave a loop-structure is to call the command exit somewhere inside the loop:

SLI ] 0
SLI [1] { 1 add dup  (Hello World) = 10 eq {exit} if }
SLI [2] loop

it prints ten times ‘Hello World’. First the initial value 0 is pushed on the operand stack. The procedure adds 1 in each cycle and takes care that one copy of the counter stays on the stack to serve as the initial value for the next cycle. After the message has been printed, the stop value 10 is pushed and is compared with the counter. If the counter is equal to 10, the nested procedure s executed. This procedure then executes the command exit, and interrupts the loop.

Command Description proc loop Repeatedly execute procedure proc. exit Exit the innermost loop structure.

Finite loops

The last example can be implemented much easier, using a repeat loop. repeat takes two arguments: An integer, and a procedure object. The integer determines how often the procedure is executed. Thus, in order to print ten times “Hello World” we write:

SLI ] 10 { (Hello World) = } repeat

Sometimes, one needs to know the counter of the loop and one may also be interested in influencing the step-size of the iterations. For this purpose SLI offers the for-loop. for is called like this:

start step stop proc for

for executes the procedure proc as long as the counter is smaller than the stop-value (for positive step values) (please refer to reference RedBook for the exact termination conditions).

In each cycle, the current value of the counter is pushed automatically. This value can be consumed by the procedure. Actually, in very long running loops, the counter must be removed by the procedure in order to avoid stack overflow. The following example prints the first ten cubic numbers:

SLI ] 1 1 10 { dup mul = } for

Command Description n proc repeat Execute procedure proc n times. i s e proc for Execute procedure proc for all values from i to e with steps. array proc forall Execute procedure proc for all elements of array. array proc forallindexed Execute procedure proc for all elements of array. array proc Map Apply proc to all elements of array. array proc MapIndexed Apply proc to all elements of array. x proc n NestList Gives a list of the results of applying proc tox 0 through n times.