The PCASTL (an acronym for ''by Parent and Childset Accessible Syntax Tree Language'') is an interpreted

high-level programming language In computer science, a high-level programming language is a programming language with strong Abstraction (computer science), abstraction from the details of the computer. In contrast to low-level programming languages, it may use natural language ...

. It was created in 2008 by Philippe Choquette. The PCASTL is designed to ease the writing of

self-modifying code In computer science, self-modifying code (SMC) is code that alters its own instructions while it is executing – usually to reduce the instruction path length and improve performance or simply to reduce otherwise repetitively similar code, ...

. The language has

reserved word In a computer language, a reserved word (also known as a reserved identifier) is a word that cannot be used as an identifier, such as the name of a variable, function, or label – it is "reserved from use". This is a syntactic definition, and a re ...

s parent and childset to access the nodes of the

syntax tree Syntax tree may refer to: * Abstract syntax tree, used in computer science * Concrete syntax tree A parse tree or parsing tree or derivation tree or concrete syntax tree is an ordered, rooted tree that represents the syntactic structure of a str ...

of the currently written code.

Hello world

The "

Hello world program ''Hello'' is a salutation or greeting in the English language. It is first attested in writing from 1826. Early uses ''Hello'', with that spelling, was used in publications in the U.S. as early as the 18 October 1826 edition of the ''Norwich C ...

" is quite simple:


"Hello, world!"


print("Hello, world!")

will do the same.

Syntax

The syntax of PCASTL is derived from

programming languages A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming ...

C and R. The source of R version 2.5.1 has been studied to write the

grammar In linguistics, the grammar of a natural language is its set of structure, structural constraints on speakers' or writers' composition of clause (linguistics), clauses, phrases, and words. The term can also refer to the study of such constraint ...

and the lexer used in the PCASTL interpreter.

Influences

Like in R, statements can, but do not have to, be separated by

semicolon The semicolon or semi-colon is a symbol commonly used as orthographic punctuation. In the English language, a semicolon is most commonly used to link (in a single sentence) two independent clauses that are closely related in thought. When a ...

s. Like in R, a

variable Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...

can change type in a session. Like in C and R, PCASTL uses balanced

bracket A bracket is either of two tall fore- or back-facing punctuation marks commonly used to isolate a segment of text or data from its surroundings. Typically deployed in symmetric pairs, an individual bracket may be identified as a 'left' or 'r ...

s () to make

block Block or blocked may refer to: Arts, entertainment and media Broadcasting * Block programming, the result of a programming strategy in broadcasting * W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...

Operators Operator may refer to: Mathematics * A symbol indicating a mathematical operation * Logical operator or logical connective in mathematical logic * Operator (mathematics), mapping that acts on elements of a space to produce elements of another sp ...

found in PCASTL have the same precedence and

associativity In mathematics, the associative property is a property of some binary operations, which means that rearranging the parentheses in an expression will not change the result. In propositional logic, associativity is a valid rule of replacement f ...

as their counterparts in C. for loops are defined like in C. ++ and --

operators Operator may refer to: Mathematics * A symbol indicating a mathematical operation * Logical operator or logical connective in mathematical logic * Operator (mathematics), mapping that acts on elements of a space to produce elements of another sp ...

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

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() # function call a()

parent and childset reserved words

Those

s can only be written lowercase and will not be recognized otherwise. The parent reserved word gives a

reference Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''name'' ...

to the parent node in the

of the code where the word is placed. In the following code, the parent node is the operator =.


a = parent

The

"a" will hold a

to the = node. The following code shows how to get

references Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''name'' ...

to the two child nodes of the operator = with the childset


a.childset a.childset

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


a
a.childset parent
a.childset parent
a.childset parent.childset 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 = `function() print("hello")'
execute(a)
a()

References

External links