PSPL

Pseudocode Styled Programming Language is a simple and experimental programming language heavily inspired from CIE styled pseudocode.

Background Information

This language is just an experimental and fun side project that can be used to evaluate simple pseudocode. While it currently does not support all the features of pseudocode from CIE specification and misses many basic features, it currently supports the following set of features:

• Variables
• Some primitive data types
  • STRING
  • INTEGER
  • FLOAT
  • BOOLEAN
• Arithmetic Expressions
• Boolean Expressions
• Basic I/O operations
• Simple conditional statements
• Loops

Following features are planned for the near future:

• Formatting support OUTPUT keyword
• Further support for selection statements
• Procedures and Functions
• Record Type
• File I/O
• Basic data structures (1D/2D Arrays)

Important Note! This is purely an experimental language that follows the vague rules of pseudocode. Many patterns in this language might not seem conventional (such as type validation in INPUT statement) so bear with it.

Installing and Usage

PSPL can be installed by simply cloning this repository. It is recommended to use Python 3.8 or a higher version.

Once cloned, simply use python -m pspl <filename> command in the cloned directory to execute the file.

$ git clone https://github.com/izxxr/pspl.git
$ cd pspl
$ python -m pspl test.pspl

Overview

Following is the basic overview of this language:

• PSPL is a dynamic language. There is no type validation at runtime. PSPL provides a DECLARE statement for declaring types of variables but that merely exists for compatiblity with pseudocode.
• OUTPUT and INPUT statements are used for writing to console and taking input respectively. For more information on their syntax, see the relevant section in documentation below.
• Variables are assigned with <- operator rather than conventional = operator. This design choice is also directly derived from pseudocode syntax. = operator is used for equality expressions.
• The rules for identifier name are traditional. No starting with numbers, only alphanumeric characters are allowed.

A basic program with simple I/O operations is below, note that DECLARE statements are not required for variables that are not used in an INPUT statement (INPUT converts the input data to declared data type) so the third line here is not required:

DECLARE n1 : INTEGER
DECLARE n2 : INTEGER
DECLARE result : INTEGER

INPUT "Number 1: ", n1
INPUT "Number 2: ", n2
result <- n1 + n2

OUTPUT "Sum: " + result

Documentation

The following is the basic documentation of PSPL.

Identifier names

• Identifier names can contain alphanumeric characters and underscores only.
• The name cannot start with a number.

Variables

Variables are assigned with <- operator rather than conventional = operator. This design choice is also directly derived from pseudocode syntax.

Example:

a <- 1
b <- 1
c <- a + b

Constants

Constants are identifiers that cannot be modified after initial definition. In order to define a constant, the CONSTANT keyword statement.

CONSTANT pi <- 3.14
CONSTANT g <- 9.81

If we try to modify a constant identifier, an error is raised:

g <- 0

At line 3, column 1, index 25:
IdentifierAlreadyDefined: Identifier 'g' has already been defined as constant

Data Types and Declaration

There are currently three primitive data types:

• String
• Integer
• Boolean

PSPL being a dynamic language does not provide type validation at runtime so there is no real use of declaring types except type conversion in INPUT statement (see it's documentation) for more information.

The DECLARE statement is used for declaring types.

Example:

DECLARE a : INTEGER

Note that attempting to use a will still result in an IdentifierNotDefined.

Strings

Strings, STRING, are well... just, strings. In order to represent a string literal, the data must be wrapped in double or single quotes.

Two strings can be concatenated to form a single string with data of both strings.

Integers

Integers, INTEGER, are simple numbers that support arithmetic operations. Nothing special.

Floating Point Numbers

Floating point numbers, FLOAT, are numbers with decimal point.

Booleans

There are two boolean literals, TRUE and FALSE of course. Boolean expressions also return a boolean.

Arithmetic Expressions

Arithmetic expressions contain mathematical operations between integers.

Following are available arithmetic operators:

• +
• -
• *
• /

Example:

a <- 1 + 1
b <- (4+2) / 3 * 2

Boolean Expressions

Boolean expressions return a boolean (TRUE or FALSE) as result.

Following are available boolean operators:

• = (Equal)
• <> (Not Equal)
• > & >
• < & <=

Example:

a <- 5
OUTPUT a > 1

Outputs TRUE.

Basic I/O operations

OUTPUT statement is used for printing an expression to the console. e.g OUTPUT "Hello World". It takes

INPUT statement can be used to take user input. This statement either takes a single operand, the identifier to store the input in or two operands with first one being the prompt to show for input and second one as identifier.

It is worth noting that if the type of identifier provided has been declared before, that type is used for implicit conversion of input. For example:

DECLARE a : INTEGER
INPUT a

Upon running above program, input will be prompted until user gives a valid integer and a will be implicity converted to integer. For booleans, case insensitive true or 1 input accounts for TRUE and vice versa.

Conditionals

IF is the only type of conditional statements currently supported. The clause must be terminated with an ENDIF.

Note that ELSE IF is not supported yet. Instead, consider using IF inside an ELSE block.

Example:

DECLARE age : INTEGER
INPUT "Age: ", age

IF age >= 18 THEN
    OUTPUT "You can drive."
ELSE
    OUTPUT "You cannot drive."
ENDIF

Loops

There are different types of loops:

• FOR loop
• DO-WHILE loop
• REPEAT-UNTIL loop

FOR loop

A for loop is used to iterate through a specific range. The basic syntax is shown in example below:

FOR a <- 1 TO 5
    OUTPUT a
ENDFOR

Above code will print numbers from 1 to 5 inclusive.

Additionally, a STEP can be added to increment or decrement numbers. For example, to print the above set of numbers in reverse order:

FOR a <- 5 TO 1 STEP -1
    OUTPUT a
ENDFOR

STEP defaults to 1.

WHILE loop

WHILE loop is used to run a block of code until a condition is TRUE.

Example of taking inputs until user does not enter the number 123:

DECLARE number : INTEGER
number <- 0

WHILE number <> 123 DO
    INPUT number
ENDWHILE

REPEAT-UNTIL loop

A repeat until loop is similar to while loop except that the condition is written after the loop body and loop body is executed at least once even if the condition is not true.

Example:

DECLARE number : INTEGER
number <- 0

REPEAT
    INPUT number
UNTIL number <> 123

In above example, even if we change second line to number <- 123, the loop body will still be executed once.

Contributing to PSPL

PSPL is an experimental and fun side project so I doubt if I will ever be seriously maintaining this so the chances of accepting major changes are really thin. Nonetheless, I will rarely be accepting pull requests that implements a new feature however bug reports and fixes are welcome.