XIDEK
Extensible Interpreter Development Kit

Baseline Languages

• Introduction
• Overview of CLL - a C-Like Language
• Overview of PLL - a Pascal-Like Language

Introduction

As a baseline for development, two simple languages were developed. The first, CLL, is a C-like language, which implements the basic expression and statement syntax of C, although it does not implement declarations, subscripts or switch statements. Why C? Many computer languages in fact use an underlying statement syntax which is similar to that of C. Also, because C is so well-known, for most people there is less new material to be learned and less explanation required.

The second language, PLL, is more like Pascal. It was created by stripping CLL of its more esoteric features and syntax and replacing them with simpler, Pascal-like constructs. PLL might well be preferred by a developer whose end users are not professional programmers.

It is particularly important to note that interpreters for both languages use the same support code. Also, the changes required for the example extensions are very nearly identical.

All the interpreters in the kit implement both integer and real arithmetic, as well as limited string capabilities. Apart from the syntactic rules for recognizing constants, these capabilities are implemented as member functions of the Value class. An extension to support complex arithmetic demonstrates the methods for extending these capabilities.

Overview of CLL - a C-Like Language

Detailed reference documentation for the CLL language syntax is provided in the CLL reference.

The CLL syntax supports the basic elements of a script language: recognition of variable names and constants, evaluation of expressions, and execution of conditional and loop statements, all based on the familiar C syntax. These may fairly be regarded as the portions of C that have become more or less standard and are not the subject of any significant controversy. Omitted are declarations, pointers, subscripts, and switch statements. The C features implemented consist of (links are to the CLL reference):

• C expression syntax
• If else statements
• While statements
• Do while statements
• For statements
• Break and continue statements
• Statement blocks
• Function calls
• Both C and C++ style comments

The CLL syntax also contains three additional elements. To illustrate changes in expression syntax, Fortran style exponentiation has been added. To aid in debugging scripts and to illustrate how to add new statements to the grammar, dump and print statements have been added.

The following features of C have not been implemented:

• Declarations, including function definitions.
• Switch statements
• Pointer arithmetic
• Arrays and subscripts
• Structs and objects

Some of the missing functionality has been implemented in three extensions to the basic syntax:

• Arrays and subscripts
• Structs and objects
• Function definitions

Since there are many ways to implement these features, all of which have their own benefits and shortcomings, these extensions are provided simply as examples and should be seen simply as suggestions as to how to proceed.

Overview of PLL - a Pascal-Like Language

To demonstrate how to modify the language to make it "simpler", XIDEK contains an example of an alternate language, PLL, many of whose features are drawn from Pascal. It will be noted that while there are substantial changes to the syntax file, interpreters for both languages can use the same support code. Here is a summary of the changes made to CLL to create PLL. The changes are described in detail in the PLL reference.

• Case Sensitivity
  Keywords and variable names are not case sensitive.
• Expression Syntax
  The complex expression syntax of C is replaced with a more Pascal-like syntax. In particular, there are far fewer precedence levels and no opportunity for side effects. Various relational and arithmetic operators have been changed to correspond to Pascal usage. Autoincrement and autodecrement operators have been removed.
• Assignment Statement Syntax
  An explicit assignment statement has been introduced, using the Pascal := operator to replace the C expression statement.
• Block Statement Syntax
  Statement blocks are delimited with the begin and end keywords instead of braces.
• If Statement Syntax
  The if statement is reconfigured to delimit the conditional expression with the keyword then instead of using parentheses.
• While Statement Syntax
  The while statement is reconfigured to delimit the conditional expression with the keyword do instead of using parentheses.
• Repeat Until Statement
  The do/while statement is replaced with a repeat/until statement.
• For Statement Syntax
  The C for statement has been replaced with a Pascal-like for statement. The downto option has not been implemented, but on the other hand, the increment may be either positive or negative.
• Break and Continue Statements
  These statements have been removed.