Eng:Programming Language

A programming language is a set of rules and functions that let people use computers, cell phones, tablets, and more devices. Programming languages are designed to make it easy for humans to write complex instructions. They function a lot like human languages: they have explicit grammars and a primitive vocabulary. Scratch is a programming language.

Theory
A programming language is a language for writing instructions for a machine. A programming language is defined by a grammar.

Many programming paradigms exist which focus on different design aspects. Imperative languages focus on commands and how to accomplish tasks, declarative languages dictate the rules and conditions of a task and do not specify the process of completing it, and functional languages emphasize functions which do not change state, and are guaranteed to always produce the same output for an input.

Languages may be designed to support paradigms, ways of programming. Paradigms include:


 * Imperative: Programs use statements
 * Functional: Programs use functions and avoid state
 * Declarative: Programs state rules of a problems
 * Object-oriented: Message passing between objects
 * Procedural: Programs are separated into subroutines

Languages usually have various primitive data types which can be expanded with Object-Oriented Programming:


 * Numbers, which are parsed as numbers
 * Some languages consider integers, floating-point numbers, and doubles to be different data types.
 * Strings, which are parsed as text
 * Arrays, which are lists of elements
 * Objects, which are dictionaries of key-value pairs.
 * Functions, which are pieces of code which can be executed with Arguments.

In essence, a programming language just provides a framework where a function can be executed with argumentsthe rest can be worked around. Usually, the grammar of a language consists of "statements", which are either:


 * Assignments: binding some value to a name (variable)
 * Procedure calls
 * Special forms: certain specialized procedures which cannot be created using the language itself, for example, IF/ELSE. The inputs to special forms are not immediately evaluated.

The syntax of a programming language gives rules about how to do each of the above. For example:


 * Assignments:
 * Squeak:
 * JavaScript:
 * Scheme:
 * Procedure calls:
 * Squeak:
 * JavaScript:
 * Scheme:
 * Special forms (if, in this case):
 * Squeak:
 * JavaScript:
 * Scheme:

Scratch simplifies programming a lot by hiding all of this in blocks: all blocks are equal, there are no special forms. Of course, in reality, certain blocks are programmed completely differently as special cases. For example, the IF block needs special programming to execute the C block contents, and the WAIT UNTIL block needs special programming to escape the atomic loop. Assignment is just another block. The REPEAT UNTIL special form block repeatedly evaluates its Boolean input, then its block, input, until the former input evaluates to true. This contrasts with custom blocks, whose inputs are evaluated prior to their execution.

Languages may be designed to support paradigms, ways of programming. Paradigms include:
 * Imperative: Programs use statements
 * Functional: Programs use functions and avoid state
 * Declarative: Programs state rules of a problem
 * Object-oriented: Message passing between objects
 * Procedural: Programs are separated into subroutines

Implementation
Programming languages are generally either interpreted or compiled, which means they are either executed directly, or translated into another language. For example, C is compiled while Python and JavaScript are interpreted. Java is compiled into bytecode, which is interpreted by the Java Virtual Machine.

The steps of a compiler can be separated into the front end and the back end. The front end translates the source language into an intermediate representation and the back end of a compiler translates the intermediate representation into the target language. This way, different compilers can be created by matching different combinations of front ends and back ends.

Language implementations usually consist of the following parts:

Front end


 * A lexer: This converts the program into tokens.
 * A parser: This analyzes the tokens as per the context-free grammar of the language, then converts them into a parse tree that can easily be interpreted.
 * A semantic analyzer: This walks the parse tree and determines the meanings of the nodes.
 * An intermediate code generator: This translates the parse tree into an intermediate representation.

Back end


 * A code optimizer: This improves the program.
 * An interpreter or compiler: The interpreter runs the program and the compiler generates an executable file.

The lexer accepts the program as an input and tokenizes it, or splits it into substrings with semantic meanings. The tokens may be defined by regular expressions. The lexer's output has two parts: the lexemes, which are the different substrings of the program, and the tokens, which are lexemes' classifications. For example, the JavaScript code  could be separated into the following tokens:

The parser analyzes the tokens and lexemes produced by the lexer program and creates an abstract syntax tree (AST). The parser utilizes a context-free grammar, the specification of the syntax. In a grammar, symbols called nonterminals are defined by productions, sequences of tokens and nonterminals. Algorithms such as LALR or Earley may be used, and the parser may be table-driven or handwritten.

Parsing algorithms are classified into top-bottom parsing, which starts with the root node and constructs the tree to the leaves, and bottom-up parsing, which start with the tokens and work up to the root node. Bottom-up parsing consists of shifts, where the next token may be pushed onto the parse stack, and reduction, where items on the stack are found to match a production, and replaced with it. Lookahead tokens are used when whether to shift or reduce is undecided.

After the parse tree is created, it may be simplified into an abstract syntax tree (AST). Then, the tree is traversed and its nodes are read. Here, the compiler may check that all the variables are declared and the data types match.

Afterwards, a lower-level intermediate representation of the program is created. GCC uses a language called RTL.

Scratch as a Programming Language
Scratch is a block-based imperative, event-driven, dynamically-typed and interpreted programming language.

Typing
Scratch is dynamically-typed, meaning that whether data types agree is checked during program execution. Scratch's primitive data types are numbers, strings and Booleans.

Special Forms
Scratch's special forms are blocks which cannot be replicated using custom blocks. These blocks may reevaluate their inputs. For example, the repeat-until loop reevaluates its Boolean input before each iteration to see if the condition's value has changed.

Implementation
Scratch is an interpreted programming language. Scratch 3.0 uses a virtual machine, which builds an abstract syntax tree.

Squeak
Squeak was used to program the 1.x series of Scratch. It is a simple language designed to be human-readable and concise. See Squeak Tutorial for a simple introduction.

Flash
Adobe Flash is a programming suite by Adobe, which was used to create Scratch 2.0. Flash programming is done in ActionScript, a language based on ECMAScript.

Python
Python is a simple interpreted scripting language that is used in the Scratch 2.0 back-end (server-side code). The Django coding platform is used on a nginx server. The forums run on DjangoBB, a Python library. Python is also used in many Scratch add-ons, including Kurt.

PHP
PHP is a programming language which was used in the Scratch website's server-side code (ScratchR) and is still used on the wiki. PHP stands for "PHP: Hypertext Preprocessor" (a ).

SQL
SQL (Structured Query Language) is a database query language. This is used to store backend information on the website, such as users and forum data.

MongoDB
MongoDB is a database system that is used to store cloud variables.

Java
Java is a powerful, object oriented programming language. Java was used to write the original Java Player for Scratch.

JavaScript
JavaScript (commonly shortened to JS) is a simple programming language based on ECMAScript for web development. Snap! is written entirely in JavaScript, and the Scratch website uses JavaScript for interactive elements. These include commenting, tagging, and editing Project Notes. In the 2.0 site, it is also used to automatically check for new messages, load What's Happening? and Scratch News, and quote posts in the forums. JavaScript is also being used to develop Scratch 3.0.

Node.js
Node.js is a backend server language based on JavaScript.

It is used on Scratch in many ways including projects, assets, cloud variables and large parts of the community.

GL Shader Language
GLSL is a programming language used in OpenGL and WebGL to process vertices and fragments. It is being used in the Scratch 3.0 renderer.

Tosh
Tosh is a cross compatible version of Scratch created by user blob8108. It allows people to use text base programming rather than the standard drag and drop method. It is designed so that people can quickly and easily create complex programs without the use of the mouse. Projects are played using a built-in version of the Phosphorus Player. The projects can also be downloaded and run on the main Scratch Website.

HTML
HTML is a web-based markup language. It is used for the layout of the Scratch website and to create the HTML5 Player.

Cascading Style Sheets
Cascading Style Sheets (commonly shortened to CSS) is a method of styling HTML documents. It is used to add color and make websites more visually appealing.