A TXL solution to [[TIL Chairmarks]] #1.3, the begin-end syntax extension for the [[Tiny Imperative Language]].

TXL is designed for implementing language extensions, so adding a new syntactic feature is simple in TXL using [[Grammar Overrides]].  The "..." in the "redefine" is not a shorthand in this example, rather a language feature of TXL that denotes an extension to the existing nonterminal alternatives.

Since there is no semantics for begin-end in TIL, we don't add any transform for this yet.

-- Main.JamesCordy - 10 Oct 2005


_File "TILbeginparser.Txl"_


<verbatim>
% TXL parser for begin-end language extenssion to Tiny Imperative Language
% Jim Cordy, October 2005

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

% Add begin-end statements using grammar overrides
redefine statement
		  ...						% refers to all existing forms for [statement]
	 |	[begin_statement]	 % add alternative for our new form
end redefine

define begin_statement
	 'begin						 [IN][NL]
		  [statement*]			[EX]
	 'end							[NL]
end define

% No need to do anything except recognize the input, since the grammar
% includes the output formatting cues
function main
	match [program]
		_ [program]
end function
</verbatim>


_Example run:_


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