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author | William Joye <wjoye@cfa.harvard.edu> | 2018-06-21 20:21:59 (GMT) |
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committer | William Joye <wjoye@cfa.harvard.edu> | 2018-06-21 20:21:59 (GMT) |
commit | 0813099a1937d0e06681609f2ef9a020ea60b451 (patch) | |
tree | 843d60b0e86bfae1be7a7ba66c4f1498c5ca07a7 /taccle/README.md | |
parent | fd4d09aafba1eb096cf9be4e93250087d9af465e (diff) | |
parent | fe273e056b3a237a2d571501d97a8d222d618047 (diff) | |
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Merge commit 'fe273e056b3a237a2d571501d97a8d222d618047' as 'taccle'
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diff --git a/taccle/README.md b/taccle/README.md new file mode 100644 index 0000000..fd10f40 --- /dev/null +++ b/taccle/README.md @@ -0,0 +1,122 @@ +Copyright +--------- +taccle itself is protected by the GNU General Public License. See the +file COPYING for details. Of course, any code you generate using +taccle is subject to whatever restrictions you place on it. + + +Availability +------------ +A wiki page for taccle may be found at http://mini.net/tcl/taccle. +An archive of taccle files are available at http://tcl.jtang.org/taccle. + + +taccle README +------------- +taccle is another compiler compiler for the Tcl language. It takes as +input a "taccle specification file"; from this taccle generates a Tcl +file that implements an LALR(1) parser. The program brings to the Tcl +language what yacc and bison do for C and C++. + +This release of taccle implements nearly all features of yacc: + - %token and %start declarations + - %define parse.error verbose declaration + - epsilon transitions (i.e., "empty" rules) + - embedded (mid-rule) actions + - literal tokens + - symbol values and synthesized attributes + - the error token and error recovery + - shift/reduce and reduce/reduce conflict notification + - operator precedence with %left, %right, %nonassoc, and %prec + - functions yyerror, YYABORT, YYACCEPT, and yyclearin + - infinite recursion detection + +Features currently missing in taccle are: + - inherited attributes + +Because taccle uses the lset command it requires Tcl 8.4 or greater. + +Be aware of the following differences between taccle and yacc: + +1. taccle (and Tcl) has no concept of variable types. Thus %union and + %type declarations are no longer needed. + +2. yacc uses variables $1, $2, etc for symbol values; it uses $$ for + the synthesized attribute. Although both $1 and $$ are legal + variable references in Tcl the latter is more awkward; the + expression `set $ foo' is not legal. Instead of $$ use the name + '_', as in `set _ foo'. These variables' scopes are valid only + within the context of an action. See + examples/simple_calculator.tac for an example. + + (The astute observer notes that using $_ for synthesized values is + similar to Perl's use of the auto-variable $_.) + +3. taccle has error handling just like bison. It always declares the + terminal `error' that is pushed onto the stack whenever it detects + a syntax error. Although yyclearin is implemented neither + YYRECOVERING nor yyerrok are. The example + interactive_calculator.tac implements error handling. + +4. Like yacc, taccle requires a token generator. The examples use + fickle (http://mini.net/tcl/fickle), though others are welcome. + +5. taccle calls a global function yylex to fetch the next token. This + yylex function is expected to return the next token in the input + stream, either a string or a number. Normally this poses no + problem except for the case of a literal zero. Tcl makes no + distinction between the integer zero and the string zero. Because + zero is reserved for the end of input marker, taccle will refuse + grammars that employ literal zeros. You can get around this by + declaring `%token ZERO' and using that instead. + +6. Another literal that causes all sorts of problems is the open brace + '{'. Because most things are represented internally as lists the + presence of the brace breaks everything. Thus taccle will refuse + to parse grammars with this literal; you'll have to use a %token + instead. + + +Using the Examples +------------------ +Execute taccle as if it were yacc: + + $ tclsh ./taccle.tcl -d some_spec_file.tac + +This will generate two files, some_spec_file.tcl and +some_spec_file.tab.tcl. The first contains the actual parser code; +execute it by calling yyparse. The latter file is file to be sourced +by the lexer, much akin to yacc's y.tab.h file. + +The Makefile in the examples subdirectory will generate two simple +calculator programs. You will need a copy of fickle (available at +http://mini.net/tcl/fickle) and may need to modify its location on +line 4 of the Makefile. After filtering simple_calculator through +ficcle and taccle execute it like so: + + $ echo "6*9" | tclsh simple_calculator.tcl + +If all goes well the program displays: + + Result is 54 + +A more interesting example is interactive_calculator.tcl. This one is +designed to handle any arbitrary number of commands. Upon errornous +equations it recovers by discarding the rest of the buffer. Here is +an example session of it: + + $ tclsh interactive_calculator.tcl + 6*9 + = 54 + 1++2++3 + = error + 1 + 2 + 3 + = 6 + +The final example, infix_calc.tcl, shows how to use operator +precedence to resolve shift/reduce conflicts. It understands both +left associative operators (addition et al) as well as right +associative (exponentation). Here is an example use: + + $ echo "2^2^2 - 10 * 2 + 6 / 3" | tclsh infix_calc.tcl + -2 |