why dose my grammar compile but crash on run

[Computermatronic]

i wrote the following grammar and generate it with asModule Metacode: Script < Statement* Operator < "+" / "-" / "_" / "\" Unary < "++" / "--" / "+" / "-" UnaryPostFix < "++" /"--" Identifier < ~([a-z_A-Z]) ~([a-zA-Z0-9]) Variable < Identifier / "(" Expression ")" Tinary < Expression "?" Expression ":" Expression ArrayInitializer < "[" (Expression ",")* "]" Expression < Unary Variable / Variable UnaryPostFix / Variable / Expression Operator Expression / Tinary / Lambda / Expression "." Identifier / Expression "[" Expression "]" / ArrayInitializer Def < "def" Type? Identifier ("=" Expression)? ";" Statement < (Expression ";" / If / For / While / Do / Foreach / Class / Function / Def ";" / Import ";" / Module ";") Block < "{" Statement* "}" / Statement If < "if" "(" Expression ")" Block For < "for" "(" Def ";" Expression ";" Expression ")" Block While < "while" "(" Expression ")" Block Do < "do" Block "until" "(" Expression ")" Foreach < "foreach" "(" (Variable ",")* Variable ";" Expression ")" Block Lambda < "function" Type? "(" (Identifier ",")* Identifier? ")" Block Function < "function" Type? Identifier "(" (Identifier ",")* Identifier? ")" Block Import < "import" Identifier ("." Identifier)* Module < "module" Identifier ("." Identifier)* Type < "<" Identifier ">"

oop stuff (currently unused)

ProtectionModifiers < "public"
                    / "package"
                    / "protected"
                    / "private" 
Class <  ProtectionModifiers? "class" Identifier ("using" (Identifier ",")* Identifier)? ClassBody 
ClassBody < "{" ClassMember* "}" 
ClassMember < ProtectionModifiers? ClassModifiers? (Def / Function) 
ClassModifiers < "static" 
AbstractClass <  ProtectionModifiers? "abstract" "class" Identifier ("using" Identifier)* AbstractClassBody 
AbstractClassBody < "{" (AbstractClassMember / ClassMember)* "}" 
AbstractClassMember < ProtectionModifiers? ClassModifiers? "abstract" "function" Type? Identifier "(" (Identifier ",")* Identifier? ")" ";" 
Interface < ProtectionModifiers "interface" Identifier InterfaceBody 
InterfaceBody < "{" InterfaceMember* "}" 
InterfaceMember < ProtectionModifiers? InterfaceModifiers? "function" Type? Identifier "(" (Identifier ",")* Identifier? ")" ";" 
InterfaceModifiers < "static" 

it compiles, but when i try to use it with module main; import metacode.parser; import std.stdio;

string test = `

class test { function f() { } } `;

int main(string[] args) { ParseTree p = Metacode(test); foreach(c;p.children) { writeln(c.name," ",c.children); foreach(d;c.children) { writeln(d.name); } } return 0; } and it crashes. why?

[PhilippeSigaud]

Hi,

mainly the crash is due to left-recursive rules: Expression < Expression Operator Expression, for example. Parsing Expression Grammars do not handle left-recursive rules, as explained in the docs. You should rewrite your rules for Expression and Tinary, so as not to have subrules beginning with Expression, else the parser enters an infinite loop (which causes the segmentation fault). As a first approach, using # to comment the left-recursive subrules in Expression (including Tinary) makes the example work.

There are two other small mistakes:

• your definition of Identifier only recognizes two-letter identifiers: *Identifier < ~([a-z_A-Z]) ~([a-z_A-Z0-9])*, you should use: Identifier < [a-z_A-Z][a-z_A-Z0-9]* to recognize any C-style identifier.
• your Operator definition has no ''? You should use Operator < "+" / "-" / "" / "\'"

Also, be aware that defining Script as Statement* allows the parsing of 0 characters as a valid parse. Hence not well-written modules will be parsed as "" (the 0-Statement option). You should maybe define Script as Statement* endOfInput to catch any mistake.

Cheers,

Philippe

[PhilippeSigaud]

Hi machinistprogrammer,

Bastiaan Veelo recently add left-recursion capability to Pegged. If you're still interested in it, you could try your grammar anew with the current HEAD.