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Re: Operator Precedence Parser

by ambrus (Abbot)
on Jun 10, 2006 at 18:58 UTC ( #554629=note: print w/replies, xml ) Need Help??

in reply to Operator Precedence Parser

I have something similar in an interpreter I started to write ages ago in perl, but has abandonned it. The interpreter works but cannot interpret user-defined functions which makes it unsuitable for most purposes. I am now not motivated to continue it.

You can download the interpreter but I also copy some relevant parts of the code here. You can download the full version and run it with -p to dump the optree instead of interpretting it. (Piping the dump through | sed 's/\bline( /\n&/g' may make it more readable.)

For this, you have to know that the get_tok function gets the next token as a string, and unget_tok pushes a token back so that it's the next token get_tok will read. The following code will parse expressions, and is called at various places from the code that parses statements.

{ my($pars_expr1,$pars_expr2); my %binops= ( # fun(args), var[index], and var.elt will be handled specially in +code "+", "add", "-", "sub", "*", "mul", "/", "div", "div", "idiv", "mod", "mod", "<", "lt", "=", "eq", ">", "gt", "<=", "le", "<>", "ne", ">=", "ge +", "es", "and", "vagy", "or", ); my %binoppr= ( "+", 50, "-", 50, "*", 40, "/", 40, "div", 40, "mod", 40, "<", 60, "=", 60, ">", 60, "<=", 60, "<>", 60, ">=", 60, "es", 40, "vagy", 50, # all operators with the same precedence are left-to-right associa +tive, # eg: a-b-c ==> (a-b)-c ); my %unops= ( # (expr), +expr are handled specially by code "-", "neg", "nem", "not", ); my %unoppr= ( "-", 30, "nem", 30, ); my %unfunc= ( "kerekit", "round", "round", "round", "egeszresz", "floor", "sqr", "sqr", "sqrt", "sqrt", ); # More precedences hard-coded in the code below: # maximal:137, unary+:30, unary():-INF, unary functions:30, # binary[]:20, binary.:20, binary():20 $pars_expr1= sub { my($t,$f,$x,$u,$p,$g); ($p,)= @_; $t= get_tok (); $t=~ m!^\d! and return [$t=~/[.eE]/?"qf":"qi", 0+$t]; is_id $t and return ["var", $t]; $f= $unops{$t} and do { $g= $unoppr{$t}; return [$f, $pars_expr2->($g<$p?$g:$p)]; }; $t eq "(" and do { $x= $pars_expr2->(137); ($u= get_tok) eq ")" or die qq!error parsing expr at "$u": ")" expected!; return $x; }; $t eq "+" and return $pars_expr2->(30<$p?30:$p); $f= $unfunc{$t} and 30<=$p and do { ($u= get_tok) eq "(" or die qq!parse error at "$u": "(" expected!; $x= $pars_expr2->(137); ($u= get_tok) eq ")" or die qq!parse error at "$u": ")" expected!; return [$f, $x]; }; $t eq "'" and return ["qs", get_tok]; die qq(error parsing expr at "$t": expr expected); }; $pars_expr2= sub { my($o,$f,$y,$x,$u,$p,$g); ($p,)= @_; $x= $pars_expr1->($p); { $o= get_tok; $f= $binops{$o} and ($g= $binoppr{$o})<=$p and do { $y= $pars_expr2->($g-1); $x= [$f, $x, $y]; redo; }; $o eq "." and 20<=$p and do { $$x[0] eq "var" or die qq(parse error at ".": expression cannot be dotted); $y= get_tok; is_id $y or die qq(parse error at "$y": record field expected); push @$x, $y; redo; }; $o eq "[" and 20<=$p and do { $$x[0] eq "var" or die qq(parse error at "[": expression cannot be subscripte +d); $y= $pars_expr2->(137); ($u= get_tok) eq "]" or die qq(parse error at "$u": "]" expected); push @$x, $y; redo; }; }; unget_tok $o; $x; }; sub pars_expr () { $pars_expr2->(137); }; sub pars_lvalue () { $pars_expr2->(20); }; };

Another interpreter I wrote in C++ as a school project also has a similar parsing routine.

Oh well, I don't think anyone would want to read this post this way. I think I'll have to make a simple example parser that works the same way as these two and post it as a meditation if I want any attention.

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