PCASTL

The PCASTL (an acronym for by Parent and Childset Accessible Syntax Tree Language) is an interpreted high-level programming language. It was created in 2008 by Philippe Choquette.^[1] The PCASTL is designed to ease the writing of self-modifying code. The language has reserved words parent and childset to access the nodes of the syntax tree of the currently written code.^[2]

Hello world[]

The "Hello world program" is quite simple:

"Hello, world!"

or

print("Hello, world!")

will do the same.

Syntax[]

The syntax of PCASTL is derived from programming languages C and R. The source of R version 2.5.1 has been studied to write the grammar and the lexer used in the PCASTL interpreter.

Influences[]

Like in R, statements can, but do not have to, be separated by semicolons.^[3] Like in R, a variable can change type in a session. Like in C and R, PCASTL uses balanced brackets ({ and }) to make blocks.

Operators found in PCASTL have the same precedence and associativity as their counterparts in C.^[2][4] for loops are defined like in C. ++ and -- operators are used like in C to increment or decrement a variable before or after it is used in its expression.

An example of PCASTL using the for reserved word and the ++ operator:

for (i = 1; i < 4; i++) print(i)

Functions and comments in PCASTL are defined like in R:

# function definition (comment)
a = function()
{
   print("Hello, world!")
}

# function call
a()

parent and childset reserved words[]

Those reserved words can only be written lowercase and will not be recognized otherwise. The parent reserved word gives a reference to the parent node in the syntax tree of the code where the word is placed. In the following code, the parent node is the operator =.

a = parent

The variable "a" will hold a reference to the = node. The following code shows how to get references to the two child nodes of the operator = with the childset reserved word.

a.childset[0]
a.childset[1]

To display the value of "a", some ways are given in this example:

a
a.childset[0].parent
a.childset[1].parent
a.childset[0].parent.childset[0].parent # and so on...

In the following code: we assign a code segment to the right child of the = node, we execute the = node a second time and we call the newly defined function.

a.childset[1] = `function() print("hello")'
execute(a)
a()

See also[]

• Abstract syntax tree
• Self-modifying code

References[]

External links[]

• PCASTL: by Parent and Childset Accessible Syntax Tree Language