Formal Languages and Translators

Internal

• Programming

TODO Deplete Parsing

Overview

Compiler

Interpreter

Syntax Analysis (Parsing)

Grammar

Derivation. The leftmost derivation. The rightmost derivation.

Production

Production head

Production body

Terminal

Nonterminal

Parse Tree

A parse tree is a graphical representation of a derivation.

Each interior node of a parse tree represents the application of a production and it is labeled with the name of the nonterminal that is the head of the production. The children node are labeled, from left to right, by the symbols in the body of the production. While is being constructed, the leaves of a parse tree may represent terminals and nonterminals, which at any moment, if they are read from left to right, they constitute the yield or the frontier of the tree, and it is also referred to as a sentential from of the grammar.

A parse tree does not contain the information that reflects the order in which productions are applied to replace nonterminals. It is customary to produce parse trees by always producing the leftmost (or rightmost) derivation. Each parse tree has associated with it a unique leftmost and an unique rightmost derivation.

Parse trees are sometimes called concrete syntax trees.

(Abstract) Syntax Tree

The abstract syntax tree, also referred to as syntax tree, represents the hierarchical syntactic structure of the parsed content, by depicting significant programming constructs. For example, in case of an expression that involves an operator, the fragment of the syntax tree that represents the expression has a node that represents the operator, and its children represent the operands. In general, any programming construct can be handled by marking up an operator for the construct and treating the semantically meaningful components of that construct as operands.

A syntax tree resembles a parse tree to a certain extent. The difference is that for a syntax tree, the internal nodes represent programming constructs, while for parse tree, the internal nodes represent nonterminals. Many nonterminals of a grammar represent programming constructs, but others are intermediate "helpers" of some sort, for example representing terms, factors or other variations of expressions. Examples of productions that are not represented in a syntax tree are expr → term or ε-productions rest → ε. These helpers are dropped into a syntax three. That is why sometimes a parse tree is called a concrete syntax tree.

Parser

Top-Down Parser

A top-down parser is a parser that employs a strategy where it first looks at the highest level of the parse tree and works down the parser tree by using the rewriting rules of the formal grammar. The top-down parser discovers the parse tree starting from the top and incrementally works its way first downwards and then rightwards. Unlike a bottom-up parser, the top-down parser eagerly decides what a construct is much earlier, when it has only scanned the leftmost symbol of that construct and has not yet parsed any of its parts.

If a language grammar has multiple rules that may start with the same leftmost symbols but have different endings, that grammar can be efficiently handled by a deterministic bottom-up parser, but cannot be handled top-down without guesswork and backtracking - so bottom-up parsers handle a somewhat larger range of computer language grammars than top-down parsers.

A recursive descent parsers is a top-down parser build from a set of possibly mutually recursive procedures, where each such procedure usually implements one of the productions of the grammar. In consequence, the structure of the resulting program closely mirrors that of the grammar it recognizes. A predictive recursive descent parser is a recursive descent parser that does not require backtracking, it is called a predictive parser.