TXL solution to [[TIL Chairmarks]] #4.3: Self-tracing program transformation.

-- Main.JamesCordy - 10 Oct 2005


_File "TILtrace.Txl"_


<verbatim>
% Simple transform to make a Tiny Imperative Language program self-tracing
% Jim Cordy, August 2005

% Replaces every statement in the program with a write statement of its text
% followed by itself.  For example:
%
%	 write ("x := 5;");
%	 x := 5;

% Begin with the TIL base grammar
include "TIL.Grm"

% We don't bother preserving comments in this transformation
% since we're not interested in maintaining the result, just running it

% Tell TXL what our string escape convention is
#pragma -esc "\"

% Allow for concise elided structured statements
redefine statement
		  ... 
	 |	  [SP] '... [SP]
end redefine

% Allow for traced statements - the attribute TRACED marks statements already done
redefine statement
		  ...
	 |	 [traced_statement]
end redefine

define traced_statement
	 [statement] [attr 'TRACED]
end define

% Replace every statement by a tracing write statement followed by itself
rule main
	 % Since our result requires two statements where one was before,
	 % we work on the statement sequence one level up
	 replace [repeat statement]
		  S [statement]
		  Rest [repeat statement]
	 % Semantic guard: if it's already done don't do it again
	 deconstruct not S
		  _ [statement] 'TRACED
	 % Make a concise version of structured statements
	 construct ConciseS [statement]
		  S [deleteBody]
	 % Make a string literal of the text of the concise statement
	 construct QuotedS [stringlit]
		  _ [+ "Trace: "] [quote ConciseS] 
	 by
		  'write QuotedS;	 'TRACED
		  S						'TRACED
		  Rest
end rule

% Replace any embedded statement sequence in the printed version of 
% a traced statement with an elision symbol for conciseness
function deleteBody
	 replace * [statement*]
		  _ [statement*]
	 by
		  '...
end function
</verbatim>


_Example run:_


<verbatim>
<linux> cat factors.til
// Factor an input number
var n;
write "Input n please";
read n;
write "The factors of n are";
var f;
f := 2;
while n != 1 do
	 while (n / f) * f = n do
		  write f;
		  n := n / f;
	 end
	 f := f + 1;
end
<linux> txl factors.til TILtrace.Txl
TXL v10.4a (15.6.05) (c)1988-2005 Queen's University at Kingston
Compiling TILtrace.Txl ... 
Parsing factors.til ...
Transforming ...
write "Trace: var n;";
var n;
write "Trace: write \"Input n please\";";
write "Input n please";
write "Trace: read n;";
read n;
write "Trace: write \"The factors of n are\";";
write "The factors of n are";
write "Trace: var f;";
var f;
write "Trace: f := 2;";
f := 2;
write "Trace: while n != 1 do ... end";
while n != 1 do
	 write "Trace: while (n / f) * f = n do ... end";
	 while (n / f) * f = n do
		  write "Trace: write f;";
		  write f;
		  write "Trace: n := n / f;";
		  n := n / f;
	 end
	 write "Trace: f := f + 1;";
	 f := f + 1;
end
<linux> 
</verbatim>