TREE-META

The TREE-META (or Tree Meta, TREEMETA) Translator Writing System is a compiler-compiler system for context-free languages originally developed in the 1960s. Parsing statements of the metalanguage resemble augmented Backus–Naur form with embedded tree-building directives. Unparsing rules include extensive tree-scanning and code-generation constructs.

History
TREE-META was instrumental in the development of NLS (oN-Line System) and was ported to many systems including the UNIVAC 1108, GE 645, SDS 940, ICL 1906A, PERQ, and UCSD p-System.

Example
This is a complete example of a TREE-META program extracted (and untested) from the more complete (declarations, conditionals, and blocks) example in Appendix 6 of the ICL 1900 TREE-META manual. That document also has a definition of TREE-META in TREE-META in Appendix 3. This program is not just a recognizer, but also outputs the assembly language for the input. It demonstrates one of the key features of TREE-META, which is tree pattern matching. It is used on both the LHS (GET and VAL for example) and the RHS (ADD and SUB).

% This is an ALGOL-style comment delimited by % % ====================== INPUT PARSE RULES ======================= %

.META PROG % A program defining driving rule is required. % % This PROG rule is the driver of the complete program. %

PROG = $STMT ; % $ is the zero or more operator. % % PROG (the program) is defined as zero or more STMT (statements). %

STMT = .ID ':=' AEXP :STORE[2]*; % Parse an assignment statement from the source to the tree. % % ':=' is a string constant, :STORE creates a STORE node,         % % [2] defines this as having two branches i.e. STORE[ID,AEXP]. % % * triggers a unparse of the tree, Starting with the last created % % tree i.e. the STORE[ID,AEXP] which is emitted as output and     % % removed from the tree. %

AEXP = FACTOR $('+' FACTOR :ADD[2] / '-' FACTOR :SUB[2]); % Here we have the recognizer for arithmetic '+' :ADD and '-' :SUB % % tree building. Again the [2] creates a 2-branch ADD or SUB tree. % % Unparsing is deferred until an entire statement has been parsed. % % ADD[FACTOR,FACTOR] or SUB[FACTOR,FACTOR]                        %

FACTOR = '-' PRIME :MINUSS[1] / PRIME ;

PRIME = .ID / .NUM / '(' AEXP ')' ?3? ; % ?3? is a hint for error messages. % % ===================== OUTPUT UNPARSE RULES ===================== %

STORE[-,-] => GET[*2] 'STORE ' *1 ; % *1 is the left tree branch. *2 is the right                     % % GET[*2] will generate code to load *2. % % The 'STORE' string will be output                               % % followed by left branch *1 a symbol                             % % Whatever *2, it will be loaded by GET[*2]. %

GET[.ID] => 'LOAD ' *1 / [.NUM] => ' LOADI ' *1 / [MINUSS[.NUM]] => 'LOADN ' *1:*1 / [-] => *1 ; % Here an .ID or a .NUM will simply be loaded. A MINUSS node      % % containing a .NUM will have this used, the notation *1:*1 means % % the first branch (a .NUM) of the first branch (MINUSS). % % Anything else will be passed on for node recognition            % % The unparse rules deconstruct a tree outputing code. %

ADD[-,-] => SIMP[*2] GET[*1] 'ADD' VAL[*2]  / SIMP[*1] GET[*2] 'ADD' VAL[*1] / GET[*1] 'STORE T+' < OUT[A] ; A<-A+1 > / GET[*2] 'ADD T+' < A<-A-1 ; OUT[A] > ; % Chevrons < > indicate an arithmetic operation, for example to   % % generate an offset A relative to a base address T.              %

SUB[-,-] => SIMP[*2] GET[*1] 'SUB' VAL[*2] / SIMP[*1] GET[*2] 'NEGATE' % 'ADD' VAL[*1] / GET[*2] 'STORE T+' < OUT[A] ; A<-A+1 > / GET[*1] 'SUB T+' < A<-A-1 ; OUT[A] > ; % A percent character in an unparse rule indicates a newline. %

SIMP[.ID]          => .EMPTY / [.NUM]         => .EMPTY / [MINUSS[.NUM]] => .EMPTY;

VAL[.ID]            => ' ' *1 / [.NUM]           => 'I ' *1 / [MINUSS[.NUM]]   => 'N ' *1:*1 ;

MINUSS[-]           => GET[*1] 'NEGATE' ;

.END