以下是我尝试为其生成解析器的EBNF格式(大部分-实际语法记录为here)语法：

expr = lambda_expr_list $;

lambda_expr_list = [ lambda_expr_list "," ] lambda_expr;

lambda_expr = conditional_expr [ "->" lambda_expr ];

conditional_expr = boolean_or_expr [ "if" conditional_expr "else" conditional_expr ];

boolean_or_expr = [ boolean_or_expr "or" ] boolean_xor_expr;

boolean_xor_expr = [ boolean_xor_expr "xor" ] boolean_and_expr;

boolean_and_expr = [ boolean_and_expr "and" ] boolean_not_expr;

boolean_not_expr = [ "not" ] relation;

relation = [ relation ( "=="
                      | "!="
                      | ">"
                      | "<="
                      | "<"
                      | ">="
                      | [ "not" ] "in"
                      | "is" [ "not" ] ) ] bitwise_or_expr;

bitwise_or_expr = [ bitwise_or_expr "|" ] bitwise_xor_expr;

bitwise_xor_expr = [ bitwise_xor_expr "^" ] bitwise_and_expr;

bitwise_and_expr = [ bitwise_and_expr "&" ] bitwise_shift_expr;

bitwise_shift_expr = [ bitwise_shift_expr ( "<<"
                                          | ">>" ) ] subtraction_expr;

subtraction_expr = [ subtraction_expr "-" ] addition_expr;

addition_expr = [ addition_expr "+" ] division_expr;

division_expr = [ division_expr ( "/"
                                | "\\" ) ] multiplication_expr;

multiplication_expr = [ multiplication_expr ( "*"
                                            | "%" ) ] negative_expr;

negative_expr = [ "-" ] positive_expr;

positive_expr = [ "+" ] bitwise_not_expr;

bitwise_not_expr = [ "~" ] power_expr;

power_expr = slice_expr [ "**" power_expr ];

slice_expr = member_access_expr { subscript };

subscript = "[" slice_defn_list "]";

slice_defn_list = [ slice_defn_list "," ] slice_defn;

slice_defn = lambda_expr
           | [ lambda_expr ] ":" [ [ lambda_expr ] ":" [ lambda_expr ] ];

member_access_expr = [ member_access_expr "." ] function_call_expr;

function_call_expr = atom { parameter_list };

parameter_list = "(" [ lambda_expr_list ] ")";

atom = identifier
     | scalar_literal
     | nary_literal;

identifier = /[_A-Za-z][_A-Za-z0-9]*/;

scalar_literal = float_literal
               | integer_literal
               | boolean_literal;

float_literal = point_float_literal
              | exponent_float_literal;

point_float_literal = /[0-9]+?\.[0-9]+|[0-9]+\./;

exponent_float_literal = /([0-9]+|[0-9]+?\.[0-9]+|[0-9]+\.)[eE][+-]?[0-9]+/;

integer_literal = dec_integer_literal
                | oct_integer_literal
                | hex_integer_literal
                | bin_integer_literal;

dec_integer_literal = /[1-9][0-9]*|0+/;

oct_integer_literal = /0[oO][0-7]+/;

hex_integer_literal = /0[xX][0-9a-fA-F]+/;

bin_integer_literal = /0[bB][01]+/;

boolean_literal = "true"
                | "false";

nary_literal = tuple_literal
             | list_literal
             | dict_literal
             | string_literal
             | byte_string_literal;

tuple_literal = "(" [ lambda_expr_list ] ")";

list_literal = "[" [ ( lambda_expr_list
                     | list_comprehension ) ] "]";

list_comprehension = lambda_expr "for" lambda_expr_list "in" lambda_expr [ "if" lambda_expr ];

dict_literal = "{" [ ( dict_element_list
                     | dict_comprehension ) ] "}";

dict_element_list = [ dict_element_list "," ] dict_element;

dict_element = lambda_expr ":" lambda_expr;

dict_comprehension = dict_element "for" lambda_expr_list "in" lambda_expr [ "if" lambda_expr ];

string_literal = /[uU]?[rR]?(\u0027(\\.|[^\\\r\n\u0027])*\u0027|\u0022(\\.|[^\\\r\n\u0022])*\u0022)/;

byte_string_literal = /[bB][rR]?(\u0027(\\[\u0000-\u007F]|[\u0000-\u0009\u000B-\u000C\u000E-\u0026\u0028-\u005B\u005D-\u007F])*\u0027|\u0022(\\[\u0000-\u007F]|[\u0000-\u0009\u000B-\u000C\u000E-\u0021\u0023-\u005B\u005D-\u007F])*\u0022)/;

我用来生成解析器的工具是Grako,它生成了一个经过修改的Packrat解析器,声称支持直接和间接的左递归.

当我在此字符串上运行生成的解析器时：

input.filter(e -> e[0] in ['t', 'T']).map(e -> (e.len().str(), e)).map(e -> '(Line length: ' + e[0] + ') ' + e[1]).list()

我收到以下错误：

grako.exceptions.FailedParse: (1:13) Expecting end of text. :
input.filter(e -> e[0] in ['t', 'T']).map(e -> (e.len().str(), e)).map(e -> '(Line length: ' + e[0] + ') ' + e[1]).list()
            ^
expr

调试显示,解析器似乎到达了第一个e [0]的末尾,然后再也没有回退到/到达它将尝试与in令牌匹配的点.

我的语法是否存在问题,以至于左递归支持的Packrat解析器将对此失败？还是应该在Grako问题追踪器上提交问题？

解决方法:

这可能是语法错误,但错误消息并未告诉您它实际发生的位置.完成语法后,我总是做的是在整个代码中嵌入cut(〜)元素(在if,operator,open括号等似乎合理的关键字之后).

cut元素使Grako生成的解析器提交到解析树中最接近的选项中采用的选项.这样,它将使解析器在实际上无法解析的表达式上报告失败,而不是让if解析器在if开始时失败.

语法中的一些错误很难发现,为此,我只是通过解析跟踪来找出解析器在输入中走了多远,以及为什么解析器无法继续下去.

我不会在PEG解析器上使用左递归进行专业工作,尽管对于简单,学术性的工作可能很好.

boolean_or_expr = boolean_xor_expr {"or" boolean_xor_expr};

然后可以在语义动作中处理关联性.

另请参阅针对Grako的issue 49下的讨论.它说,用于支持左递归的算法将不会总是在最终的AST中产生预期的关联性.