Slide 2.1: Programming language syntax

Programming Language Syntax

Syntax refers to the ways symbols may be combined to create well-formed sentences (or programs) in the language. Syntax defines the formal relations between the constituents of a language, thereby providing a structural description of the various expressions that make up legal strings in the language. Syntax deals solely with the form and structure of symbols in a language without any consideration given to their meaning.

Grammars and BNF
Defining the syntax of programming languages bears a close resemblance to formulating the grammar of a natural language, describing how symbols may be formed into the valid phrases of the language. A grammar <Σ, N, P, S> consists of four parts:

A finite set Σ of terminal symbols, the alphabet of the language, that are assembled to make up the sentences in the language.

A finite set N of nonterminal symbols or syntactic categories, each of which represents some collection of subphrases of the sentences. A nonterminal is represented with the form “<category-name>”.

A finite set P of productions or rules that describe how each nonterminal is defined in terms of terminal symbols and nonterminals. A production is represented with the form:
```
   <stmt> ::= if <expr> then <stmt>
```
where “if” and “then” are terminal symbols in the language. The symbol “::=” can be read “is defined to be.”

A distinguished nonterminal S, the start symbol, that specifies the principal category being defined—for example, sentence or program.

When applied to programming languages, this notation is known as Backus-Naur Form or BNF.