TILParser Using TXL

Software Transformation Systems
TXL solution for TIL Chairmarks #1: A parser for the Tiny Imperative Language (TIL) implemented in TXL.

This is the entire solution, run using the command "txl program.til TILparser.Txl" no other libraries, support modules or data files are needed. See below for an example run.

This version accepts but does not preserve comments, which is the default in TXL. For a pretty-printer for TIL see TIL Pretty Printer Using TXL.

-- JamesCordy - 17 Aug 2005 (revised JamesCordy - 10 Oct 2005)

File "TILparser.Txl"

% TXL parser / pretty-printer for Tiny Imperative Language
% Jim Cordy, April 2005

% All TXL parsers are automatically also pretty-printers if the
% grammar includes the optional formatting cues, as in this case

% Use the TIL grammar shown below
include "TIL.Grm"

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

File "TIL.Grm"

% TXL grammar for Tiny Imperative Language
% Jim Cordy, April 2005

% Note that all quoting, as in 'if, is not required,
% rather an optional coding convention used by TXL programmers

% Formatting cues, such as [NL] on the right, are optional output formatting
% suggestions and have no input parse meaning

% From the spec, it is not clear if TIL should be priority expression parsed or not,
% so this grammar provides both, controlled by this switch
% #define PRIORITY

% Keywords of TIL, assuming TIL is a reserved-word language
% If not, remove this section
    var if then else while do for read write
end keys

% Compound tokens to be recognized as a single lexical unit
    :=  !=
end compounds

% Commenting convention for TIL, assuming it has one
end comments

% Direct TXL encoding of the TIL specification grammar -
% I don't think any explantions are needed from here on
% [NL], [IN] and [EX] on the right are optional pretty-printing cues

define program
end define

define statement
    |   [assignment_statement]
    |   [if_statement]
    |   [while_statement]
    |   [for_statement]
    |   [read_statement]
    |   [write_statement]
end define

% Untyped variables
define declaration
    'var [id] ;                   [NL]
end define

define assignment_statement
    [id] := [expression] ;        [NL]
end define

define if_statement
    'if [expression] 'then        [IN][NL]
        [statement*]              [EX]
    [opt else_statement]
    'end                          [NL]
end define

define else_statement
    'else                        [IN][NL]
        [statement*]             [EX]
end define

% While loop 
define while_statement
    'while [expression] 'do      [IN][NL]
        [statement*]             [EX]
    'end                         [NL]
end define

% Declaring for
define for_statement
    'for [id] := [expression] 'to [expression] 'do      [IN][NL]
        [statement*]                                    [EX]
    'end                                                [NL]
end define

define read_statement
    'read [id] ;                 [NL]
end define

define write_statement
    'write [expression] ;        [NL]
end define

#if not PRIORITY then
% Simple nonpriority expression grammar, as specified
define expression
    |   [expression] [op] [expression]
end define

define op 
        = | !=       
    |   + | -
    |   * | /        
end define

% Alternative traditional priority expression grammar
define expression
    |   [expression] [eqop] [term]
end define

define eqop
        = | !=
end define

define term
    |   [term] [addop] [factor]
end define

define addop
        + | -
end define

define factor
    |   [factor] [mulop] [primary]
end define

define mulop
        * | /
end define
#end if PRIORITY

define primary
    |   [literal]
    |   ( [expression] )
end define

define literal
    |   [stringlit]
end define

Example run:

(Note comments are accepted in input and removed in output; see TIL Pretty Printer Using TXL for a comment-preserving version.)

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