Recursive descent parser generator
Write a recursive descent parser generator that takes a description of a grammar as input and outputs the source code for a parser in the same language as the generator. (So a generator written in C++ would output C++ source code for the parser.) You can assume that all of the rules have been preprocessed into a form suitable for the construction of a recursive descent parser. Check the following links for more details.
- http://www.cs.engr.uky.edu/~lewis/essays/compilers/rec-des.html
- http://www.engr.mun.ca/~theo/Misc/exp_parsing.htm
Use the parser generator and a grammar file to build a parser that takes an arithmetic expression and turns it in to three address code. The resulting parser should take this (or something similar) as input:
(one + two) * three - four * five
And generate this (or something similar) as output:
_0001 = one + two _0002 = _0001 * three _0003 = four * five _0004 = _0002 - _0003
C++
This program translates an annotated LL(1) grammar into a C++ lexer plus parser. Each rule is required to return a string of some kind and the return values of the already matched nonterminals (and matched text of terminals) can be accessed with $1, $2, etc. which are replaced by the appropriate string variable.
It can't handle newlines as part of the grammar, the error checking is fairly limited and the error reporting is basically non-existent, but the parser it generates (not shown below) is human readable.
<lang cpp>
- include <iostream>
- include <fstream>
- include <string>
- include <sstream>
- include <map>
- include <set>
- include <regex>
using namespace std;
map<string, string> terminals; map<string, vector<vector<string>>> nonterminalRules; map<string, set<string>> nonterminalFirst; map<string, vector<string>> nonterminalCode;
int main(int argc, char **argv) { if (argc < 3) { cout << "Usage: <input file> <output file>" << endl; return 1; }
ifstream inFile(argv[1]); ofstream outFile(argv[2]);
regex blankLine(R"(^\s*$)"); regex terminalPattern(R"((\w+)\s+(.+))"); regex rulePattern(R"(^!!\s*(\w+)\s*->\s*((?:\w+\s*)*)$)"); regex argPattern(R"(\$(\d+))"); smatch results;
// Read terminal patterns string line; while (true) { getline(inFile, line);
// Terminals section ends with a blank line if (regex_match(line, blankLine)) break;
regex_match(line, results, terminalPattern); terminals[results[1]] = results[2]; }
outFile << "#include <iostream>" << endl; outFile << "#include <fstream>" << endl; outFile << "#include <string>" << endl; outFile << "#include <regex>" << endl; outFile << "using namespace std;" << endl << endl;
// Generate the token processing functions outFile << "string input, nextToken, nextTokenValue;" << endl; outFile << "string prevToken, prevTokenValue;" << endl << endl;
outFile << "void advanceToken() {" << endl; outFile << " static smatch results;" << endl << endl;
outFile << " prevToken = nextToken;" << endl; outFile << " prevTokenValue = nextTokenValue;" << endl << endl;
for (auto i = terminals.begin(); i != terminals.end(); ++i) { string name = i->first + "_pattern"; string pattern = i->second;
outFile << " static regex " << name << "(R\"(^\\s*(" << pattern << "))\");" << endl; outFile << " if (regex_search(input, results, " << name << ", regex_constants::match_continuous)) {" << endl; outFile << " nextToken = \"" << i->first << "\";" << endl; outFile << " nextTokenValue = results[1];" << endl; outFile << " input = regex_replace(input, " << name << ", \"\");" << endl; outFile << " return;" << endl; outFile << " }" << endl << endl; }
outFile << " static regex eof(R\"(\\s*)\");" << endl; outFile << " if (regex_match(input, results, eof, regex_constants::match_continuous)) {" << endl; outFile << " nextToken = \"\";" << endl; outFile << " nextTokenValue = \"\";" << endl; outFile << " return;" << endl; outFile << " }" << endl << endl;
outFile << " throw \"Unknown token\";" << endl; outFile << "}" << endl << endl;
outFile << "bool same(string symbol) {" << endl; outFile << " if (symbol == nextToken) {" << endl; outFile << " advanceToken();" << endl; outFile << " return true;" << endl; outFile << " }" << endl; outFile << " return false;" << endl; outFile << "}" << endl << endl;
// Copy the header code to the output while (true) { getline(inFile, line);
// Copy lines until we reach the first rule if (regex_match(line, results, rulePattern)) break;
outFile << line << endl; }
// Build the nonterminal table while (true) { // results already contains the last matched rule string name = results[1]; stringstream ss(results[2]);
string tempString; vector<string> tempVector; while (ss >> tempString) tempVector.push_back(tempString); nonterminalRules[name].push_back(tempVector);
// Read code until another rule is found string code = ""; while (true) { getline(inFile, line);
if (!inFile || regex_match(line, results, rulePattern)) break;
// Replace $1 with results[1], etc. line = regex_replace(line, argPattern, "results[$1]");
code += line + "\n"; } nonterminalCode[name].push_back(code);
// Stop when we reach the end of the file if (!inFile) break; }
// Generate the first sets, inefficiently bool done = false; while (!done) for (auto i = nonterminalRules.begin(); i != nonterminalRules.end(); ++i) { string name = i->first; done = true;
if (nonterminalFirst.find(i->first) == nonterminalFirst.end()) nonterminalFirst[i->first] = set<string>();
for (int j = 0; j < i->second.size(); ++j) { if (i->second[j].size() == 0) nonterminalFirst[i->first].insert(""); else { string first = i->second[j][0]; if (nonterminalFirst.find(first) != nonterminalFirst.end()) { for (auto k = nonterminalFirst[first].begin(); k != nonterminalFirst[first].end(); ++k) { if (nonterminalFirst[name].find(*k) == nonterminalFirst[name].end()) { nonterminalFirst[name].insert(*k); done = false; } } } else if (nonterminalFirst[name].find(first) == nonterminalFirst[name].end()) { nonterminalFirst[name].insert(first); done = false; } } } }
// Generate function signatures for the nonterminals for (auto i = nonterminalRules.begin(); i != nonterminalRules.end(); ++i) { string name = i->first + "_rule"; outFile << "string " << name << "();" << endl; } outFile << endl;
// Generate the nonterminal functions for (auto i = nonterminalRules.begin(); i != nonterminalRules.end(); ++i) { string name = i->first + "_rule"; outFile << "string " << name << "() {" << endl; outFile << " vector<string> results;" << endl; outFile << " results.push_back(\"\");" << endl << endl;
// Check if this rule can match an empty string int epsilon = -1; for (int j = 0; epsilon == -1 && j < i->second.size(); ++j) if (i->second[j].size() == 0) epsilon = j;
// Generate each production for (int j = 0; j < i->second.size(); ++j) { // Nothing to generate for an empty rule if (j == epsilon) continue;
string token = i->second[j][0]; if (terminals.find(token) != terminals.end()) outFile << " if (nextToken == \"" << i->second[j][0] << "\") {" << endl; else { outFile << " if ("; bool first = true; for (auto k = nonterminalFirst[token].begin(); k != nonterminalFirst[token].end(); ++k, first = false) { if (!first) outFile << " || "; outFile << "nextToken == \"" << (*k) << "\""; } outFile << ") {" << endl; }
for (int k = 0; k < i->second[j].size(); ++k) { if (terminals.find(i->second[j][k]) != terminals.end()) { outFile << " if(same(\"" << i->second[j][k] << "\"))" << endl; outFile << " results.push_back(prevTokenValue);" << endl; outFile << " else" << endl; outFile << " throw \"Syntax error - mismatched token\";" << endl; } else outFile << " results.push_back(" << i->second[j][k] << "_rule());" << endl; }
// Copy rule code to output outFile << nonterminalCode[i->first][j];
outFile << " }" << endl << endl; }
if (epsilon == -1) outFile << " throw \"Syntax error - unmatched token\";" << endl; else outFile << nonterminalCode[i->first][epsilon];
outFile << "}" << endl << endl; }
// Generate the main function outFile << "int main(int argc, char **argv) {" << endl; outFile << " if(argc < 2) {" << endl; outFile << " cout << \"Usage: <input file>\" << endl;" << endl; outFile << " return 1;" << endl; outFile << " }" << endl << endl;
outFile << " ifstream file(argv[1]);" << endl; outFile << " string line;" << endl; outFile << " input = \"\";" << endl << endl;
outFile << " while(true) {" << endl; outFile << " getline(file, line);" << endl; outFile << " if(!file) break;" << endl; outFile << " input += line + \"\\n\";" << endl; outFile << " }" << endl << endl;
outFile << " advanceToken();" << endl << endl;
outFile << " start_rule();" << endl; outFile << "}" << endl; } </lang>
Example grammar:
plus \+
minus -
times \*
div /
open \(
close \)
num [0-9]+
var [a-z]+
string nextLabel() {
static string label = "0000";
for(int i = label.length() - 1; i >= 0; --i) {
if(label[i] == '9')
label[i] = '0';
else {
++label[i];
break;
}
}
return "_" + label;
}
!! start -> expr start2
if($2 == "")
return $1;
else {
string label = nextLabel();
cout << label << " = " << $1 << " " << $2 << endl;
return label;
}
!! start2 -> plus start
return "+ " + $2;
!! start2 -> minus start
return "- " + $2;
!! start2 ->
return "";
!! expr -> term expr2
if($2 == "")
return $1;
else {
string label = nextLabel();
cout << label << " = " << $1 << " " << $2 << endl;
return label;
}
!! expr2 -> times expr
return "* " + $2;
!! expr2 -> div expr
return "/ " + $2;
!! expr2 ->
return "";
!! term -> var
return $1;
!! term -> num
return $1;
!! term -> open start close
return $2;
Example input to parser (filename passed through command line):
(one + two) * three + four * five
Output (to standard out):
_0001 = one + two _0002 = _0001 * three _0003 = four * five _0004 = _0002 + _0003
Go
The standard library already contains a recursive descent parser for Go programs or expressions, written in Go itself, whose output is an abstract syntax tree (AST) representing such code.
I've therefore applied this parser to the expression designated by the task, which only involves binary expressions and identifiers, and written a separate routine to convert the resulting AST into three-address code. <lang go>package main
import (
"fmt" "go/ast" "go/parser" "log"
)
func labelStr(label int) string {
return fmt.Sprintf("_%04d", label)
}
type binexp struct {
op, left, right string kind, index int
}
func main() {
x := "(one + two) * three - four * five"
fmt.Println("Expression to parse: ", x)
f, err := parser.ParseExpr(x)
if err != nil {
log.Fatal(err)
}
fmt.Println("\nThe abstract syntax tree for this expression:")
ast.Print(nil, f)
fmt.Println("\nThe corresponding three-address code:")
var binexps []binexp
// Inspect nodes in depth-first order.
ast.Inspect(f, func(n ast.Node) bool {
switch x := n.(type) {
case *ast.BinaryExpr:
sx, ok1 := x.X.(*ast.Ident)
sy, ok2 := x.Y.(*ast.Ident)
op := x.Op.String()
if ok1 && ok2 {
binexps = append(binexps, binexp{op, sx.Name, sy.Name, 3, 0})
} else if !ok1 && ok2 {
binexps = append(binexps, binexp{op, "<addr>", sy.Name, 2, 0})
} else if ok1 && !ok2 {
binexps = append(binexps, binexp{op, sx.Name, "<addr>", 1, 0})
} else {
binexps = append(binexps, binexp{op, "<addr>", "<addr>", 0, 0})
}
}
return true
})
for i := 0; i < len(binexps); i++ {
binexps[i].index = i
}
label, last := 0, -1
var ops, args []binexp
var labels []string
for i, be := range binexps {
if be.kind == 0 {
ops = append(ops, be)
}
if be.kind != 3 {
continue
}
label++
ls := labelStr(label)
fmt.Printf(" %s = %s %s %s\n", ls, be.left, be.op, be.right)
for j := i - 1; j > last; j-- {
be2 := binexps[j]
if be2.kind == 2 {
label++
ls2 := labelStr(label)
fmt.Printf(" %s = %s %s %s\n", ls2, ls, be2.op, be2.right)
ls = ls2
be = be2
} else if be2.kind == 1 {
label++
ls2 := labelStr(label)
fmt.Printf(" %s = %s %s %s\n", ls2, be2.left, be2.op, ls)
ls = ls2
be = be2
}
}
args = append(args, be)
labels = append(labels, ls)
lea, leo := len(args), len(ops)
for lea >= 2 {
if i < len(binexps)-1 && args[lea-2].index <= ops[leo-1].index {
break
}
label++
ls2 := labelStr(label)
fmt.Printf(" %s = %s %s %s\n", ls2, labels[lea-2], ops[leo-1].op, labels[lea-1])
ops = ops[0 : leo-1]
args = args[0 : lea-1]
labels = labels[0 : lea-1]
lea--
leo--
args[lea-1] = be
labels[lea-1] = ls2
}
last = i
}
}</lang>
- Output:
Expression to parse: (one + two) * three - four * five
The abstract syntax tree for this expression:
0 *ast.BinaryExpr {
1 . X: *ast.BinaryExpr {
2 . . X: *ast.ParenExpr {
3 . . . Lparen: 1
4 . . . X: *ast.BinaryExpr {
5 . . . . X: *ast.Ident {
6 . . . . . NamePos: 2
7 . . . . . Name: "one"
8 . . . . . Obj: *ast.Object {
9 . . . . . . Kind: bad
10 . . . . . . Name: ""
11 . . . . . }
12 . . . . }
13 . . . . OpPos: 6
14 . . . . Op: +
15 . . . . Y: *ast.Ident {
16 . . . . . NamePos: 8
17 . . . . . Name: "two"
18 . . . . . Obj: *(obj @ 8)
19 . . . . }
20 . . . }
21 . . . Rparen: 11
22 . . }
23 . . OpPos: 13
24 . . Op: *
25 . . Y: *ast.Ident {
26 . . . NamePos: 15
27 . . . Name: "three"
28 . . . Obj: *(obj @ 8)
29 . . }
30 . }
31 . OpPos: 21
32 . Op: -
33 . Y: *ast.BinaryExpr {
34 . . X: *ast.Ident {
35 . . . NamePos: 23
36 . . . Name: "four"
37 . . . Obj: *(obj @ 8)
38 . . }
39 . . OpPos: 28
40 . . Op: *
41 . . Y: *ast.Ident {
42 . . . NamePos: 30
43 . . . Name: "five"
44 . . . Obj: *(obj @ 8)
45 . . }
46 . }
47 }
The corresponding three-address code:
_0001 = one + two
_0002 = _0001 * three
_0003 = four * five
_0004 = _0002 - _0003
J
J's native recursive descent parser is adequate for this task, if we map names appropriately.
Implementation:
<lang J>cocurrent 'base'
inlocale=: 4 :0 L:0
x,'_',y,'_'
)
parse=: 3 :0
sentence=. ;:y opinds=. (;:'+*-')i.sentence opfuns=. (;:'plus times minus') inlocale 'base' scratch=. cocreate coinsert__scratch 'base' names=. ~.sentence#~_1<:nc sentence (names inlocale scratch)=: names r=. do__scratch ;:inv opinds}((#sentence)#"0 opfuns),sentence codestroy__scratch r
)
term=: 1 :0
2 :('m,m,expr n')
)
expr=:1 :0
r=. genname emit r,'=:',x,m,y r
)
plus=: '+' expr times=: '*' term minus=: '-' expr
N=: 10000 genname=: 3 :0
'z',}.":N=: N+1
)
emit=: smoutput </lang>
Task example:
<lang J> parse '(one + two) * three - four * five' z0001=:four*five z0002=:one+two z0003=:z0002*three z0004=:z0003-z0001 z0004</lang>
See also https://github.com/jsoftware/general_misc/blob/master/trace.ijs for a model of the underlying parser being employed here.
Julia
The Julia compiler's own parser is a recursive descent parser, and can be used directly here. <lang julia>const one, two, three, four, five, six, seven, eight, nine = collect(1:9)
function testparser(s)
cod = Meta.parse(s) println(Meta.lower(Main, cod))
end
testparser("(one + two) * three - four * five")
</lang>
- Output:
$(Expr(:thunk, CodeInfo(
@ none within `top-level scope'
1 ─ %1 = one + two
│ %2 = %1 * three
│ %3 = four * five
│ %4 = %2 - %3
└── return %4
)))