Translational Backus–Naur form explained

Translational Backus–Naur Form (TBNF or Translational BNF) refers to Backus–Naur form, which is a formal grammar notation used to define the syntax of computer languages, such as Algol, Ada, C++, COBOL, Fortran, Java, Perl, Python, and many others. TBNF goes beyond BNF and extended BNF (EBNF) grammar notation because it not only defines the syntax of a language, but also defines the structure of the abstract syntax tree (AST) to be created in memory and the output intermediate code to be generated. Thus TBNF defines the complete translation process from input source code to intermediate code. Specification of the output intermediate code is optional, in which case you will still get automatic AST creation and have the ability to define its structure in the grammar.

Overview

The TBNF concept was first published in April 2006 in a paper at SIGPLAN Notices, a special interest group of the ACM.^[1]

Here is a sample grammar specified in TBNF: /* TBNF Grammar for a simple language. Five node arguments are used in this grammar to avoid having to create node actions.

/* Input Tokens. */

=> error ; => lookup; // Lookup & store in symbol table. => lookup; // Lookup & store in symbol table. ;

/* Operator precedence. */

<< // Lowest priority. << << // Highest priority.

/* Productions. */

Goal -> Program... *> goal_ (0,,"\t\tSTART\n",,"\t\tEOF\n\n") Program -> 'program' '' *> program_ (2,,"\t\tPROGRAM %s\n",,"\t\tEND PROGRAM %s\n") Stmt -> Assignment -> IfThen -> IfElse -> IfThenElse Assignment ~> Target '=' Exp ';' *> assign_ (0,,,,"\t\tSTORE\n") IfThen -> 'if' RelExp Then 'endif' *> if_ (0,,"if&0:\n",,"endif&0:\n") IfElse -> 'if' RelExp Else 'endif' *> if_ (0,,"if&0:\n",,"endif&0:\n") IfThenElse -> 'if' RelExp Then2 Else2 'endif' *> if_ (0,,"if&0:\n",,"endif&0:\n") Target -> *> ident_ (1,,,,"\t\tLADR %s\n") RelExp -> Exp '

' Exp > eq_ (0,,,,"\t\tEQ\n") -> Exp '!=' Exp > ne_ (0,,,,"\t\tNE\n") Exp -> Primary -> Exp '+' Exp > add_ (0,,,,"\t\tADD\n") -> Exp '-' Exp > sub_ (0,,,,"\t\tSUB\n") -> Exp '' Exp > mul_ (0,,,,"\t\tMUL\n") -> Exp '/' Exp > div_ (0,,,,"\t\tDIV\n") Primary -> > intr_ (1,,,,"\t\tLOAD %s\n") -> > ident_ (1,,,,"\t\tLOAD %s\n") -> '(' Exp ')' Then -> 'then' Stmt... > then_ (0,,"\t\tBR NZ endif&1\nthen&1:\n",,) Else -> 'else' Stmt... > else_ (0,,"\t\tBR Z endif&1\nelse&1:\n",,) Then2 -> 'then' Stmt... > then2_ (0,,"\t\tBR NZ else&1\nthen&1:\n",,) Else2 -> 'else' Stmt... *> else2_ (0,,"\t\tBR endif&1\nelse&1:\n",,)

/* End of Grammar. */

Given this input:

program test

Running the translator generated from the above grammar would produce this output:

START PROGRAM testif1: LOAD a LOAD 0 EQ BR NZ else1then1:if2: LOAD x LOAD 0 EQ BR NZ else2then2: LOAD 10 LADR b STORE BR endif2else2: LOAD 20 LADR b STOREendif2: BR endif1else1:if3: LOAD x LOAD 1 EQ BR NZ else3then3: LOAD 30 LADR b STORE BR endif3else3: LOAD 40 LADR b STOREendif3:endif1: END PROGRAM test EOF

Notes and References

Mann . Paul B . 2006 . A Translational BNF Grammar Notation (TBNF) . SIGPLAN Notices . 4 . 41 . 16–23 . 10.1145/1147214.1147218 .