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cpp-peglib
==========
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C++17 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.
Since this library only supports C++17 compilers, please make sure that compiler the option `-std=c++17` is enabled. (`/std:c++17 /Zc:__cplusplus` for MSVC)
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) by Bryan Ford. *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)
* `\u10FFFF` (Unicode char)
* `%whitespace` (Automatic whitespace skipping)
* `%word` (Word expression)
* `$name(` ... `)` (Capture scope operator)
* `$name<` ... `>` (Named capture operator)
* `$name` (Backreference operator)
* `|` (Dictionary operator)
* `↑` (Cut operator)
* `MACRO_NAME(` ... `)` (Parameterized rule or Macro)
* `{ precedence L - + L / * }` (Parsing infix expression)
* `%recovery(` ... `)` (Error recovery operator)
* `exp⇑label` or `exp^label` (Syntax sugar for `(exp / %recover(label))`)
* `label { message "..." }` (Error message instruction)
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 semantic 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(static_cast<bool>(parser) == true);
// (3) Setup actions
parser["Additive"] = [](const SemanticValues &vs) {
switch (vs.choice()) {
case 0: // "Multitive '+' Additive"
return any_cast<int>(vs[0]) + any_cast<int>(vs[1]);
default: // "Multitive"
return any_cast<int>(vs[0]);
}
};
parser["Multitive"] = [](const SemanticValues &vs) {
switch (vs.choice()) {
case 0: // "Primary '*' Multitive"
return any_cast<int>(vs[0]) * any_cast<int>(vs[1]);
default: // "Primary"
return any_cast<int>(vs[0]);
}
};
parser["Number"] = [](const SemanticValues &vs) {
return vs.token_to_number<int>();
};
// (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& vs, any& dt)
[](const SemanticValues& vs)
[](SemanticValues& vs, any& dt)
[](SemanticValues& vs)
```
`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 choice'
`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.
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& vs` 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_view sv() const { return sv_; }
// Line number and column at which the matched string is
std::pair<size_t, size_t> line_info() const;
// Tokens
std::vector<std::string_view> tokens;
std::string_view token(size_t id = 0) const;
// Token conversion
std::string token_to_string(size_t id = 0) const;
template <typename T> T token_to_number() 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& vs) {
// 'token' doesn't include trailing whitespaces
auto token = vs.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& vs) {
assert(vs.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& vs) {
auto val = vs.token_to_number<long>();
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 available.
```cpp
parser["RULE"].enter = [](const char* s, size_t n, any& dt) {
std::cout << "enter" << std::endl;
};
parser["RULE"] = [](const SemanticValues& vs, 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' | '...'
```
Cut operator
------------
`↑` operator could mitigate backtrack performance problem, but has a risk to change the meaning of grammar.
```peg
S <- '(' ↑ P ')' / '"' ↑ P '"' / P
P <- 'a' / 'b' / 'c'
```
When we parse `(z` with the above grammar, we don't have to backtrack in `S` after `(` is matched, because a cut operator is inserted there.
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& vs) -> long {
auto result = any_cast<long>(vs[0]);
if (vs.size() > 1) {
auto ope = any_cast<char>(vs[1]);
auto num = any_cast<long>(vs[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& vs) { return *vs.sv(); };
parser["NUMBER"] = [](const SemanticValues& vs) { return vs.token_to_number<long>(); };
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.
NOTE: An AST node holds a corresponding token as `std::string_vew` for performance and less memory usage. It is users' responsibility to keep the original source text along with the generated AST tree.
```
peg::parser parser(R"(
...
defenition1 <- ... { no_ast_opt }
defenition2 <- ... { no_ast_opt }
...
)");
parser.enable_ast();
shared_ptr<peg::Ast> ast;
if (parser.parse("...", ast)) {
cout << peg::ast_to_s(ast);
ast = parser.optimize_ast(ast);
cout << peg::ast_to_s(ast);
}
```
`optimize_ast` removes redundant nodes to make a AST simpler. If you want to disable this behavior from particular rules, `no_ast_opt` instruction can be used.
It internally calls `peg::AstOptimizer` to do the job. 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 making a parser by parsing PEG syntax text, we can also construct a parser by hand with *parser combinators*. 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& vs) {
tags.push_back(vs.token_to_string());
};
_ <= zom(cls(" \t"));
auto ret = ROOT.parse(" [tag1] [tag:2] [tag-3] ");
```
The following are available operators:
| Operator | Description | 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 |
| rec | 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& vs, 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????`.
Error report and recovery
-------------------------
cpp-peglib supports the furthest failure error position report as described in the Bryan Ford original document.
For better error report and recovery, cpp-peglib supports 'recovery' operator with label which can be associated with a recovery expression and a custom error message. This idea comes from the fantastic ["Syntax Error Recovery in Parsing Expression Grammars"](https://arxiv.org/pdf/1806.11150.pdf) paper by Sergio Medeiros and Fabio Mascarenhas.
The custom message supports `%t` which is a place holder for the unexpected token, and `%c` for the unexpected Unicode char.
Here is an example of Java-like grammar:
```peg
# java.peg
Prog ← 'public' 'class' NAME '{' 'public' 'static' 'void' 'main' '(' 'String' '[' ']' NAME ')' BlockStmt '}'
BlockStmt ← '{' (!'}' Stmt^stmtb)* '}' # Annotated with `stmtb`
Stmt ← IfStmt / WhileStmt / PrintStmt / DecStmt / AssignStmt / BlockStmt
IfStmt ← 'if' '(' Exp ')' Stmt ('else' Stmt)?
WhileStmt ← 'while' '(' Exp^condw ')' Stmt # Annotated with `condw`
DecStmt ← 'int' NAME ('=' Exp)? ';'
AssignStmt ← NAME '=' Exp ';'^semia # Annotated with `semi`
PrintStmt ← 'System.out.println' '(' Exp ')' ';'
Exp ← RelExp ('==' RelExp)*
RelExp ← AddExp ('<' AddExp)*
AddExp ← MulExp (('+' / '-') MulExp)*
MulExp ← AtomExp (('*' / '/') AtomExp)*
AtomExp ← '(' Exp ')' / NUMBER / NAME
NUMBER ← < [0-9]+ >
NAME ← < [a-zA-Z_][a-zA-Z_0-9]* >
%whitespace ← [ \t\n]*
%word ← NAME
# Recovery operator labels
semia ← '' { message "missing semicolon in assignment." }
stmtb ← (!(Stmt / 'else' / '}') .)* { message "invalid statement" }
condw ← &'==' ('==' RelExp)* / &'<' ('<' AddExp)* / (!')' .)*
```
For instance, `';'^semi` is a syntactic sugar for `(';' / %recovery(semi))`. `%recover` operator tries to recover the error at ';' by skipping input text with the recovery expression `semi`. Also `semi` is associated with a custom message "missing semicolon in assignment.".
Here is the result:
```java
> cat sample.java
public class Example {
public static void main(String[] args) {
int n = 5;
int f = 1;
while( < n) {
f = f * n;
n = n - 1
};
System.out.println(f);
}
}
> peglint java.peg sample.java
sample.java:5:12: syntax error, unexpected '<', expecting <NAME>, <NUMBER>, <WhileStmt>.
sample.java:8:5: missing semicolon in assignment.
sample.java:8:6: invalid statement
```
As you can see, it can now show more than one error, and provide more meaningful error messages than the default messages.
peglint - PEG syntax lint utility
---------------------------------
### Build peglint
```
> cd lint
> mkdir build
> cd build
> cmake ..
> make
> ./peglint
usage: grammar_file_path [source_file_path]
options:
--source: source text
--packrat: enable packrat memoise
--ast: show AST tree
--opt, --opt-all: optimize all AST nodes except nodes selected with `no_ast_opt` instruction
--opt-only: optimize only AST nodes selected with `no_ast_opt` instruction
--trace: show trace messages
```
### Grammar check
```
> cat a.peg
Additive <- Multitive '+' Additive / Multitive
Multitive <- Primary '*' Multitive / Primary
Primary <- '(' Additive ')' / Number
%whitespace <- [ \t\r\n]*
> peglint a.peg
[commendline]:3:35: 'Number' is not defined.
```
### Source check
```
> 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
```
### AST
```
> 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)
```
### AST optimazation
```
> peglint --ast --opt --source "1 + 2 * 3" a.peg
+ Additive
- Multitive[Number] (1)
+ Additive[Multitive]
- Primary[Number] (2)
- Multitive[Number] (3)
```
### Adjust AST optimazation with `no_ast_opt` instruction
```
> cat a.peg
Additive <- Multitive '+' Additive / Multitive
Multitive <- Primary '*' Multitive / Primary
Primary <- '(' Additive ')' / Number { no_ast_opt }
Number <- < [0-9]+ >
%whitespace <- [ \t\r\n]*
> peglint --ast --opt --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 --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)
* [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)
License
-------
MIT license (© 2021 Yuji Hirose)
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