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First of all, are the Semantic Rules and Abstract Syntax Tree Rules the same?
Now, if i have a language specifications, and i have CFG, then how do i go about constructing Abstract Syntax Tree Rules. Any source is appreciated. Thanks.
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An Abstract Syntax Tree describes the parse tree logically. It does not need to contain all the syntactical constructs required to parse some source code (white spaces, braces, keywords, parenthesis etc). That's why Parse Tree is also called Concrete Syntax Tree while the AST is called Syntax Tree .
Abstract Syntax Tree is a kind of tree representation of the abstract syntactic structure of source code written in a programming language. Each node of the tree denotes a construct occurring in the source code.
A syntax tree is a tree in which each leaf node represents an operand, while each inside node represents an operator. The Parse Tree is abbreviated as the syntax tree. The syntax tree is usually used when representing a program in a tree structure.
CST(Concrete Syntax Tree) is a tree representation of the Grammar(Rules of how the program should be written). Depending on compiler architecture, it can be used by the Parser to produce an AST. AST(Abstract Syntax Tree) is a tree representation of Parsed source, produced by the Parser part of the compiler.
"Abstract Syntax Tree" Rules (this is strange terminology) might be interpreted as those rules that shape the construction of the abstract syntax as parsing proceeds. These are usually written, in a grammar rule for a nonterminal T, as constructors over abstract syntax trees produced by parsing the subsidiary phrases of T. If
T = '(' A ';' B ')' ;
is a grammar rule, an AST constructor for T might be
T(A,B)
implying the construction of a T node with children being the ASTs constructed for the A and B subparses.
Semantic Rules are constraints that the program must meet to be legal, beyond the mere syntax. So one can construct an abstract syntax tree (from "rules"); doing so only demonstrates the program is syntactically correct. But the abstract syntax can say things that are simply nonsensical semantically, e.g.,
"declare s as function; ... s=7; ..."
The only way to check this in general is to walk over the abstract syntax tree, collecting facts locally (e.g., "s is a function" is a fact extracted from the declare statement; "s is assigned an integer" is collected from the assignment) and propagating those facts until the they meet and are shown to be (in)compatible.
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