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cpp-peglib/README.md
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cpp-peglib
==========
[![Build Status](https://travis-ci.org/yhirose/cpp-peglib.svg?branch=master)](https://travis-ci.org/yhirose/cpp-peglib)
[![Bulid Status](https://ci.appveyor.com/api/projects/status/github/yhirose/cpp-peglib?branch=master&svg=true)](https://ci.appveyor.com/project/yhirose/cpp-peglib)
C++11 header-only [PEG](http://en.wikipedia.org/wiki/Parsing_expression_grammar) (Parsing Expression Grammars) library. You can start using it right away just by including `peglib.h` in your project.
You can also try the online version, PEG Playground at https://yhirose.github.io/cpp-peglib.
The PEG syntax is well described on page 2 in the [document](http://www.brynosaurus.com/pub/lang/peg.pdf). *cpp-peglib* also supports the following additional syntax for now:
* `'...'i` (Case-insensitive literal operator)
* `[^...]` (Negated character class operator)
* `{2,5}` (Regex-like repetition operator)
* `<` ... `>` (Token boundary operator)
* `~` (Ignore operator)
* `\x20` (Hex number char)
* `%whitespace` (Automatic whitespace skipping)
* `%word` (Word expression)
* `$name(` ... `)` (Capture scope operator)
* `$name<` ... `>` (Named capture operator)
* `$name` (Backreference operator)
* `|` (Dictionary operator)
* `MACRO_NAME(` ... `)` (Parameterized rule or Macro)
* `{ precedence L - + L / * }` (Parsing infix expression)
This library supports the linear-time parsing known as the [*Packrat*](http://pdos.csail.mit.edu/~baford/packrat/thesis/thesis.pdf) parsing.
IMPORTANT NOTE for some Linux distributions such as Ubuntu and CentOS: Need `-pthread` option when linking. See [#23](https://github.com/yhirose/cpp-peglib/issues/23#issuecomment-261126127), [#46](https://github.com/yhirose/cpp-peglib/issues/46#issuecomment-417870473) and [#62](https://github.com/yhirose/cpp-peglib/issues/62#issuecomment-492032680).
How to use
----------
This is a simple calculator sample. It shows how to define grammar, associate samantic actions to the grammar, and handle semantic values.
```cpp
// (1) Include the header file
#include <peglib.h>
#include <assert.h>
#include <iostream>
using namespace peg;
using namespace std;
int main(void) {
// (2) Make a parser
parser parser(R"(
# Grammar for Calculator...
Additive <- Multitive '+' Additive / Multitive
Multitive <- Primary '*' Multitive / Primary
Primary <- '(' Additive ')' / Number
Number <- < [0-9]+ >
%whitespace <- [ \t]*
)");
assert((bool)parser == true);
// (3) Setup actions
parser["Additive"] = [](const SemanticValues& sv) {
switch (sv.choice()) {
case 0: // "Multitive '+' Additive"
return any_cast<int>(sv[0]) + any_cast<int>(sv[1]);
default: // "Multitive"
return any_cast<int>(sv[0]);
}
};
parser["Multitive"] = [](const SemanticValues& sv) {
switch (sv.choice()) {
case 0: // "Primary '*' Multitive"
return any_cast<int>(sv[0]) * any_cast<int>(sv[1]);
default: // "Primary"
return any_cast<int>(sv[0]);
}
};
parser["Number"] = [](const SemanticValues& sv) {
return stoi(sv.token(), nullptr, 10);
};
// (4) Parse
parser.enable_packrat_parsing(); // Enable packrat parsing.
int val;
parser.parse(" (1 + 2) * 3 ", val);
assert(val == 9);
}
```
To show syntax errors in grammar text:
```cpp
auto grammar = R"(
# Grammar for Calculator...
Additive <- Multitive '+' Additive / Multitive
Multitive <- Primary '*' Multitive / Primary
Primary <- '(' Additive ')' / Number
Number <- < [0-9]+ >
%whitespace <- [ \t]*
)";
parser parser;
parser.log = [](size_t line, size_t col, const string& msg) {
cerr << line << ":" << col << ": " << msg << "\n";
};
auto ok = parser.load_grammar(grammar);
assert(ok);
```
There are four semantic actions available:
```cpp
[](const SemanticValues& sv, any& dt)
[](const SemanticValues& sv)
[](SemanticValues& sv, any& dt)
[](SemanticValues& sv)
```
`SemanticValues` value contains the following information:
- Semantic values
- Matched string information
- Token information if the rule is literal or uses a token boundary operator
- Choice number when the rule is 'prioritized choise'
`any& dt` is a 'read-write' context data which can be used for whatever purposes. The initial context data is set in `peg::parser::parse` method.
`peg::any` is a simpler implementatin of std::any. If the compiler in use supports C++17, by default `peg::any` is defined as an alias to `std::any`.
To force using the simpler `any` implementation that comes with `cpp-peglib`, define `PEGLIB_USE_STD_ANY` as 0 before including `peglib.h`:
```cpp
#define PEGLIB_USE_STD_ANY 0
#include <peglib.h>
[...]
```
A semantic action can return a value of arbitrary data type, which will be wrapped by `peg::any`. If a user returns nothing in a semantic action, the first semantic value in the `const SemanticValues& sv` argument will be returned. (Yacc parser has the same behavior.)
Here shows the `SemanticValues` structure:
```cpp
struct SemanticValues : protected std::vector<any>
{
// Input text
const char* path;
const char* ss;
// Matched string
std::string str() const; // Matched string
const char* c_str() const; // Matched string start
size_t length() const; // Matched string length
// Line number and column at which the matched string is
std::pair<size_t, size_t> line_info() const;
// Tokens
std::vector<
std::pair<
const char*, // Token start
size_t>> // Token length
tokens;
std::string token(size_t id = 0) const;
// Choice number (0 based index)
size_t choice() const;
// Transform the semantic value vector to another vector
template <typename T> vector<T> transform(size_t beg = 0, size_t end = -1) const;
}
```
The following example uses `<` ... ` >` operator, which is *token boundary* operator.
```cpp
peg::parser parser(R"(
ROOT <- _ TOKEN (',' _ TOKEN)*
TOKEN <- < [a-z0-9]+ > _
_ <- [ \t\r\n]*
)");
parser["TOKEN"] = [](const SemanticValues& sv) {
// 'token' doesn't include trailing whitespaces
auto token = sv.token();
};
auto ret = parser.parse(" token1, token2 ");
```
We can ignore unnecessary semantic values from the list by using `~` operator.
```cpp
peg::parser parser(R"(
ROOT <- _ ITEM (',' _ ITEM _)*
ITEM <- ([a-z])+
~_ <- [ \t]*
)");
parser["ROOT"] = [&](const SemanticValues& sv) {
assert(sv.size() == 2); // should be 2 instead of 5.
};
auto ret = parser.parse(" item1, item2 ");
```
The following grammar is same as the above.
```cpp
peg::parser parser(R"(
ROOT <- ~_ ITEM (',' ~_ ITEM ~_)*
ITEM <- ([a-z])+
_ <- [ \t]*
)");
```
*Semantic predicate* support is available. We can do it by throwing a `peg::parse_error` exception in a semantic action.
```cpp
peg::parser parser("NUMBER <- [0-9]+");
parser["NUMBER"] = [](const SemanticValues& sv) {
auto val = stol(sv.str(), nullptr, 10);
if (val != 100) {
throw peg::parse_error("value error!!");
}
return val;
};
long val;
auto ret = parser.parse("100", val);
assert(ret == true);
assert(val == 100);
ret = parser.parse("200", val);
assert(ret == false);
```
*enter* and *leave* actions are also avalable.
```cpp
parser["RULE"].enter = [](const char* s, size_t n, any& dt) {
std::cout << "enter" << std::endl;
};
parser["RULE"] = [](const SemanticValues& sv, any& dt) {
std::cout << "action!" << std::endl;
};
parser["RULE"].leave = [](const char* s, size_t n, size_t matchlen, any& value, any& dt) {
std::cout << "leave" << std::endl;
};
```
Ignoring Whitespaces
--------------------
As you can see in the first example, we can ignore whitespaces between tokens automatically with `%whitespace` rule.
`%whitespace` rule can be applied to the following three conditions:
* trailing spaces on tokens
* leading spaces on text
* trailing spaces on literal strings in rules
These are valid tokens:
```
KEYWORD <- 'keyword'
KEYWORDI <- 'case_insensitive_keyword'
WORD <- < [a-zA-Z0-9] [a-zA-Z0-9-_]* > # token boundary operator is used.
IDNET <- < IDENT_START_CHAR IDENT_CHAR* > # token boundary operator is used.
```
The following grammar accepts ` one, "two three", four `.
```
ROOT <- ITEM (',' ITEM)*
ITEM <- WORD / PHRASE
WORD <- < [a-z]+ >
PHRASE <- < '"' (!'"' .)* '"' >
%whitespace <- [ \t\r\n]*
```
Word expression
---------------
```cpp
peg::parser parser(R"(
ROOT <- 'hello' 'world'
%whitespace <- [ \t\r\n]*
%word <- [a-z]+
)");
parser.parse("hello world"); // OK
parser.parse("helloworld"); // NG
```
Capture/Backreference
---------------------
```cpp
peg::parser parser(R"(
ROOT <- CONTENT
CONTENT <- (ELEMENT / TEXT)*
ELEMENT <- $(STAG CONTENT ETAG)
STAG <- '<' $tag< TAG_NAME > '>'
ETAG <- '</' $tag '>'
TAG_NAME <- 'b' / 'u'
TEXT <- TEXT_DATA
TEXT_DATA <- ![<] .
)");
parser.parse("This is <b>a <u>test</u> text</b>."); // OK
parser.parse("This is <b>a <u>test</b> text</u>."); // NG
parser.parse("This is <b>a <u>test text</b>."); // NG
```
Dictionary
----------
`|` operator allows us to make a word dictionary for fast lookup by using Trie structure internally. We don't have to worry about the order of words.
```peg
START <- 'This month is ' MONTH '.'
MONTH <- 'Jan' | 'January' | 'Feb' | 'February' | '...'
```
Parameterized Rule or Macro
---------------------------
```peg
# Syntax
Start ← _ Expr
Expr ← Sum
Sum ← List(Product, SumOpe)
Product ← List(Value, ProOpe)
Value ← Number / T('(') Expr T(')')
# Token
SumOpe ← T('+' / '-')
ProOpe ← T('*' / '/')
Number ← T([0-9]+)
~_ ← [ \t\r\n]*
# Macro
List(I, D) ← I (D I)*
T(x) ← < x > _
```
Parsing infix expression by Precedence climbing
-----------------------------------------------
Regarding the *precedence climbing algorithm*, please see [this article](https://eli.thegreenplace.net/2012/08/02/parsing-expressions-by-precedence-climbing).
```cpp
parser parser(R"(
EXPRESSION <- INFIX_EXPRESSION(ATOM, OPERATOR)
ATOM <- NUMBER / '(' EXPRESSION ')'
OPERATOR <- < [-+/*] >
NUMBER <- < '-'? [0-9]+ >
%whitespace <- [ \t]*
# Declare order of precedence
INFIX_EXPRESSION(A, O) <- A (O A)* {
precedence
L + -
L * /
}
)");
parser["INFIX_EXPRESSION"] = [](const SemanticValues& sv) -> long {
auto result = any_cast<long>(sv[0]);
if (sv.size() > 1) {
auto ope = any_cast<char>(sv[1]);
auto num = any_cast<long>(sv[2]);
switch (ope) {
case '+': result += num; break;
case '-': result -= num; break;
case '*': result *= num; break;
case '/': result /= num; break;
}
}
return result;
};
parser["OPERATOR"] = [](const SemanticValues& sv) { return *sv.c_str(); };
parser["NUMBER"] = [](const SemanticValues& sv) { return atol(sv.c_str()); };
long val;
parser.parse(" -1 + (1 + 2) * 3 - -1", val);
assert(val == 9);
```
*precedence* instruction can be applied only to the following 'list' style rule.
```
Rule <- Atom (Operator Atom)* {
precedence
L - +
L / *
R ^
}
```
*precedence* instruction contains precedence info entries. Each entry starts with *associativity* which is 'L' (left) or 'R' (right), then operator tokens follow. The first entry has the highest order level.
AST generation
--------------
*cpp-peglib* is able to generate an AST (Abstract Syntax Tree) when parsing. `enable_ast` method on `peg::parser` class enables the feature.
```
peg::parser parser("...");
parser.enable_ast();
shared_ptr<peg::Ast> ast;
if (parser.parse("...", ast)) {
cout << peg::ast_to_s(ast);
std::vector<std::string> exceptions = { "defenition1", "defenition2 };
ast = peg::AstOptimizer(true, exceptions).optimize(ast);
cout << peg::ast_to_s(ast);
}
```
`peg::AstOptimizer` removes redundant nodes to make a AST simpler. You can make your own AST optimizers to fit your needs.
See actual usages in the [AST calculator example](https://github.com/yhirose/cpp-peglib/blob/master/example/calc3.cc) and [PL/0 language example](https://github.com/yhirose/cpp-peglib/blob/master/pl0/pl0.cc).
Make a parser with parser combinators
-------------------------------------
Instead of makeing a parser by parsing PEG syntax text, we can also construct a parser by hand with *parser combinatorss*. Here is an example:
```cpp
using namespace peg;
using namespace std;
vector<string> tags;
Definition ROOT, TAG_NAME, _;
ROOT <= seq(_, zom(seq(chr('['), TAG_NAME, chr(']'), _)));
TAG_NAME <= oom(seq(npd(chr(']')), dot())), [&](const SemanticValues& sv) {
tags.push_back(sv.str());
};
_ <= zom(cls(" \t"));
auto ret = ROOT.parse(" [tag1] [tag:2] [tag-3] ");
```
The following are available operators:
| Operator | Description |
| :------- | :------------------------------ |
| seq | Sequence |
| cho | Prioritized Choice |
| zom | Zero or More |
| oom | One or More |
| opt | Optional |
| apd | And predicate |
| npd | Not predicate |
| lit | Literal string |
| liti | Case-insensitive Literal string |
| cls | Character class |
| ncls | Negated Character class |
| chr | Character |
| dot | Any character |
| tok | Token boundary |
| ign | Ignore semantic value |
| csc | Capture scope |
| cap | Capture |
| bkr | Back reference |
| dic | Dictionary |
| pre | Infix expression |
| usr | User defined parser |
Adjust definitions
------------------
It's possible to add/override definitions.
```cpp
auto syntax = R"(
ROOT <- _ 'Hello' _ NAME '!' _
)";
Rules additional_rules = {
{
"NAME", usr([](const char* s, size_t n, SemanticValues& sv, any& dt) -> size_t {
static vector<string> names = { "PEG", "BNF" };
for (const auto& name: names) {
if (name.size() <= n && !name.compare(0, name.size(), s, name.size())) {
return name.size(); // processed length
}
}
return -1; // parse error
})
},
{
"~_", zom(cls(" \t\r\n"))
}
};
auto g = parser(syntax, additional_rules);
assert(g.parse(" Hello BNF! "));
```
Unicode support
---------------
cpp-peglib accepts UTF8 text. `.` matches a Unicode codepoint. Also, it supports `\u????`.
peglint - PEG syntax lint utility
---------------------------------
### Build peglint
```
> cd lint
> mkdir build
> cd build
> cmake ..
> make
> ./peglint
usage: grammar_file_path [source_file_path]
options:
--ast: show AST tree
--opt, --opt-all: optimaze all AST nodes except nodes selected with --opt-rules
--opt-only: optimaze only AST nodes selected with --opt-rules
--opt-rules rules: comma delimitted definition rules for optimazation
--source: source text
--trace: show trace messages
```
### Lint grammar
```
> cat a.peg
A <- 'hello' ^ 'world'
> peglint a.peg
a.peg:1:14: syntax error
```
```
> cat a.peg
A <- B
> peglint a.peg
a.peg:1:6: 'B' is not defined.
```
```
> cat a.peg
A <- B / C
B <- 'b'
C <- A
> peglint a.peg
a.peg:1:10: 'C' is left recursive.
a.peg:3:6: 'A' is left recursive.
```
### Lint source text
```
> cat a.peg
Additive <- Multitive '+' Additive / Multitive
Multitive <- Primary '*' Multitive / Primary
Primary <- '(' Additive ')' / Number
Number <- < [0-9]+ >
%whitespace <- [ \t\r\n]*
> peglint --source "1 + a * 3" a.peg
[commendline]:1:3: syntax error
```
```
> cat a.txt
1 + 2 * 3
> peglint --ast a.peg a.txt
+ Additive
+ Multitive
+ Primary
- Number (1)
+ Additive
+ Multitive
+ Primary
- Number (2)
+ Multitive
+ Primary
- Number (3)
```
```
> peglint --ast --opt --source "1 + 2 * 3" a.peg
+ Additive
- Multitive[Number] (1)
+ Additive[Multitive]
- Primary[Number] (2)
- Multitive[Number] (3)
```
```
> peglint --ast --opt --opt-rules "Primary" --source "1 + 2 * 3" a.peg
+ Additive/0
+ Multitive/1[Primary]
- Number (1)
+ Additive/1[Multitive]
+ Primary/1
- Number (2)
+ Multitive/1[Primary]
- Number (3)
```
```
> peglint --ast --opt-only --opt-rules "Primary" --source "1 + 2 * 3" a.peg
+ Additive/0
+ Multitive/1
- Primary/1[Number] (1)
+ Additive/1
+ Multitive/0
- Primary/1[Number] (2)
+ Multitive/1
- Primary/1[Number] (3)
```
Sample codes
------------
* [Calculator](https://github.com/yhirose/cpp-peglib/blob/master/example/calc.cc)
* [Calculator (with parser operators)](https://github.com/yhirose/cpp-peglib/blob/master/example/calc2.cc)
* [Calculator (AST version)](https://github.com/yhirose/cpp-peglib/blob/master/example/calc3.cc)
* [Calculator (parsing expressions by precedence climbing)](https://github.com/yhirose/cpp-peglib/blob/master/example/calc4.cc)
* [Calculator (AST version and parsing expressions by precedence climbing)](https://github.com/yhirose/cpp-peglib/blob/master/example/calc5.cc)
* [Monkey language](https://github.com/yhirose/monkey-cpp) described in [Writing An Interpreter In Go](https://interpreterbook.com/).
* [PL/0 language example](https://github.com/yhirose/cpp-peglib/blob/master/pl0/pl0.cc)
* [A tiny PL/0 JIT compiler in less than 700 LOC with LLVM and PEG parser](https://github.com/yhirose/pl0-jit-compiler)
* [A Programming Language just for writing Fizz Buzz program. :)](https://github.com/yhirose/fizzbuzzlang)
PEG debug
---------
A debug viewer for Parsing Expression Grammars using cpp-peglib by [mqnc](https://github.com/mqnc). Please see [his gihub project page](https://github.com/mqnc/pegdebug) for the detail. You can see a parse result of PL/0 code [here](https://mqnc.github.io/pegdebug/example/output.html).
License
-------
MIT license (© 2020 Yuji Hirose)
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