For-to-Nondeclaring-ForUsingTXL

TXL solution to TIL Chairmarks #2.1, declaring "for" statement to nondeclaring "for" statement.

-- JamesCordy - 10 Oct 2005

File "TILfordeclare.Txl"

% TXL transformation to adapt existing "declaring for" semantics TIL programs
% to a proposed new semantics in which "for" control variables must be declared
% Jim Cordy, October 2005

% Note: In this program all keywords of TIL are quoted in patterns and replacements -
% this is not strictly necessary, but is conventional style in modern TXL programs.

% Based on the TIL base grammar
include "TIL.Grm"

% Preserve comments, we're probably going to maintain the result
include "TILCommentOverrides.Grm"

% Transformation rule to correct all for loops.

% Default rewrite rule semantics in TXL are compositional fixed point,
% which for most transformations is the desired semantics.  However,
% this transformation has no syntactic compositional fixed point.
% So the alternatives are either a semantically-guarded transformation,
% a grammar override, or an explicit once-over traversal.
% This solution uses the last of these, not because it is the best solution
% but because this example gives us an opportunity to demonstrate that possibility.

function main
    % This is a function transformer, so it applies only once unless explicitly
    % reapplied.  The "replace *" allows it to search deeply for its pattern.
    % The replacement reapplies it twice to implement an explicit traversal.

    replace * [statement*]
        'for Id [id] := Expn1 [expression] 'to Expn2 [expression] 'do
            Statements [statement*]
        'end
        MoreStatements [statement*]
    by
        'var Id;
        'for Id := Expn1 'to Expn2 'do
            Statements [main]
        'end
        MoreStatements [main]
end function

Example run:

<linux> cat multiples.til 
// Test of declaring for loop in TIL
// Output first 10 multiples of numbers 1 through 9
for i := 1 to 9 do
for j := 1 to 10 do
    write i*j;
end end
<linux> txl multiples.til TILfordeclare.Txl 
TXL v10.4a (15.6.05) (c)1988-2005 Queen's University at Kingston
Compiling TILfordeclare.Txl ... 
Parsing multiples.til ...
Transforming ...
// Test of declaring for loop in TIL
// Output first 10 multiples of numbers 1 through 9
var i;
for i := 1 to 9 do
    var j;
    for j := 1 to 10 do
        write i * j;
    end
end
<linux>