# PCASTL

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High-level programming language

PCASTL Paradigm imperative, reflective Designed by Philippe Choquette Developer Philippe Choquette First appeared 2008; 18 years ago (2008) Stable release 3.5 / March 31, 2018; 8 years ago (2018-03-31) OS Cross-platform License GNU Lesser General Public License Website www.pcosmos.ca/pcastl/ Influenced by C, R

The **PCASTL** (an acronym for *by Parent and Childset Accessible Syntax Tree Language*) is an interpreted [high-level programming language](/source/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](/source/Self-modifying_code). The language has [reserved words](/source/Reserved_word) **parent** and **childset** to access the nodes of the [syntax tree](/source/Abstract_syntax_tree) of the currently written code.[2]

## Hello world

The "[Hello world program](/source/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](/source/Programming_languages) [C](/source/C_(programming_language)) and [R](/source/R_(programming_language)). The source of [R](/source/R_(programming_language)) version 2.5.1 has been studied to write the [grammar](/source/Formal_grammar) and the [lexer](/source/Lexical_analyzer) used in the PCASTL interpreter.

### Influences

Like in [R](/source/R_(programming_language)), statements can, but do not have to, be separated by [semicolons](/source/Semicolon).[3] Like in [R](/source/R_(programming_language)), a [variable](/source/Variable_(programming)) can change type in a [session](/source/Session_(computer_science)). Like in [C](/source/C_(programming_language)) and [R](/source/R_(programming_language)), PCASTL uses balanced [brackets](/source/Bracket) (**{** and **}**) to make [blocks](/source/Block_(programming)).

[Operators](/source/Operator_(programming)) found in PCASTL have the same [precedence](/source/Order_of_operations) and [associativity](/source/Operator_associativity) as their counterparts in [C](/source/C_(programming_language)).[2][4] **for** loops are defined like in [C](/source/C_(programming_language)). ++ and -- [operators](/source/Operator_(programming)) are used like in [C](/source/C_(programming_language)) to increment or decrement a variable before or after it is used in its expression.

An example of PCASTL using the **for** [reserved word](/source/Reserved_word) and the ++ [operator](/source/Operator_(programming)):

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

[Functions](/source/Subroutine) and [comments](/source/Comment_(computing)) in PCASTL are defined like in [R](/source/R_(programming_language)):

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

# function call
a()

### parent and childset reserved words

Those [reserved words](/source/Reserved_word) can only be written lowercase and will not be recognized otherwise. The **parent** reserved word gives a [reference](/source/Reference_(computer_science)) to the parent node in the [syntax tree](/source/Abstract_syntax_tree) of the code where the word is placed. In the following code, the parent node is the [operator](/source/Operator_(programming)) =.

a = parent

The [variable](/source/Variable_(programming)) "a" will hold a [reference](/source/Reference_(computer_science)) to the = node. The following code shows how to get [references](/source/Reference_(computer_science)) to the two child nodes of the [operator](/source/Operator_(programming)) = with the **childset** [reserved word](/source/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

- [Free and open-source software portal](https://en.wikipedia.org/wiki/Portal:Free_and_open-source_software)

- [Abstract syntax tree](/source/Abstract_syntax_tree)

- [Self-modifying code](/source/Self-modifying_code)

## References

1. **[^](#cite_ref-1)** ["pcosmos.ca"](http://www.pcosmos.ca/). Philippe Choquette. Retrieved 2008-06-14.

1. ^ [***a***](#cite_ref-PCASTLmain_2-0) [***b***](#cite_ref-PCASTLmain_2-1) ["PCASTL: by Parent and Childset Accessible Syntax Tree Language"](http://www.pcosmos.ca/pcastl/). Philippe Choquette. Retrieved 2008-06-14.

1. **[^](#cite_ref-3)** ["An Introduction to R"](https://cran.r-project.org/doc/manuals/R-intro.html). R Development Core Team. Retrieved 2008-06-14.

1. **[^](#cite_ref-4)** Hanly, Jeri R.; Elliot B. Koffman (1999). [*Problem Solving & Program Design in C, Third Edition*](https://archive.org/details/problemsolvingpr00hanl_1). [Addison-Wesley](/source/Addison-Wesley). [ISBN](/source/ISBN_(identifier)) [0-201-35748-8](https://en.wikipedia.org/wiki/Special:BookSources/0-201-35748-8).

## External links

- [PCASTL: by Parent and Childset Accessible Syntax Tree Language](http://www.pcosmos.ca/pcastl/)

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Adapted from the Wikipedia article [PCASTL](https://en.wikipedia.org/wiki/PCASTL) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/PCASTL?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.