Writing IntelliJ Idea plugin . Part 4 - Taming grammar recursion

In the beginning our parser was pretty simple, but progrramming language is complicated, has operators and also expressions. Brute force approach does not work well for big files due to recursion.

Finishing parser - Taming recursion

Our parser is finally finished, our terminal symbols are in place (defined in parser and resolved via lexer regular expressions) - but some grammar constructs are not yet adequately covered. Our language has chained function invocations, and expressions (not as complex as java, but nevertheless they need coverage and operator precedence and whatever). Our base expression is:

term ::=
         // any primitive object
         object |
         // method can be involved with square params on implicit object,
         // or with round on first
         ( METHOD (squareParams|roundParams)?) |
         // can always have brackets around for precedence
         (PAREN_OPEN expression PAREN_CLOSE) |
         // lambda
         lambda |
         // function invocation -  up to 15 params - we are not  in grammar
         ( HASH_INVOCATION squareParams?) |

Important part is treatment of expression in parenthesises as primtive term - this way we ensure that parenthesis has precedence over everything. Now, as everything is object for us, we can call methods on them (highest precedence):

invocation_chain ::= invocation_chain dot_invocation | term  

This would work, produce nice parsing tree for us, but also introduce recursion - recursion is bad for performance. We shall use following instead:

invocation_chain ::= term  dot_invocation*

(exact syntax of dot invocation is not of interest here, but it contains parameters which are in turn expressions - it’s turtles all the way down, and cost of recursion would be prohibitive). We also have postfix operator with highest precedence (factorial), which can be applied multiple times - and we also do not like to have any recursion here:

postfix_expression ::=  invocation_chain FACTORIAL*

Then prefix operators, which have lesser precedence. We can not avoid recursion here - but since we have primitive prefix this is not as expensive:

unary_expression ::=
           // prefix
           ( MINUS | PLUS | NOT ) unary_expression |

Now, as we have covered postfix and prefix we shall cover infix operators - which come in 4 flavours (decreasing prcedence):

  • Multiplicative ( * / )

  • Additive ( + -)

  • Boolean and ( and, comparison operators )

  • boolean or ( or, xor)

    Naive recursive attempt to solve this would be:

mutiplicative_exception ::= unary_expression MULT_OP mutiplicative_exception | unary_expression
additive_expression ::= mutiplicative_exception (PLUS| MINUS) additive_expression | mutiplicative_exception
compare_and_expression ::=  additive_expression (BOOL_AND|COMP_OP) compare_and_expression | additive_expression
bool_or_expression ::=   compare_and_expression BOOL_OR bool_or_expression | compare_and_expression

Looks clean and almost like in java grammar, lesser preference delegates to higher level - but performance for more complicated expressions is way below expectations (it takes several minutes for bigger files) - not a good solution. But rhis can be reworked avoiding recursion:

mutiplicative_expression ::= unary_expression (MULT_OP unary_expression)*
additive_expression ::= mutiplicative_expression ( (PLUS| MINUS) mutiplicative_expression)*
compare_and_expression ::=  additive_expression ((BOOL_AND|COMP_OP)  additive_expression)*
bool_or_expression ::=   compare_and_expression (BOOL_OR  compare_and_expression)*

This grammar would not produce nice PSI trees, but it works way faster than recursive one. Now just hang our rule chain to expression (staring from lowest possible precedence):

expression ::=

And your parser is way faster and copes with all the long files.