Lesson 18 · AND, OR, and precedence

AND, OR, and precedence

Real filters combine conditions, and AND binds tighter than OR. Today you parse boolean expressions so that the tree reflects the correct precedence without any parentheses.

The goal

Parse AND and OR combinations of comparisons into a tree where AND binds tighter than OR.

Start here - the target

TO DO

Scenario: Parsing boolean operators with precedence

Giventhe expression "a = 1 AND b = 2 OR c = 3"

Whenit is parsed

Thenthe top node is an OR whose left is (a = 1 AND b = 2) and right is (c = 3)

And"a = 1 OR b = 2 AND c = 3" parses with the AND grouped on the right of the OR

Background

WHERE a = 1 AND b = 2 OR c = 3 has to mean (a = 1 AND b = 2) OR c = 3 - AND binds more tightly than OR, just like * binds tighter than + in arithmetic. The classic recursive-descent way to encode this is one function per precedence level: an OR parser that calls an AND parser for each of its operands, and an AND parser that calls the comparison parser for each of its.

Because the outer level is the lowest-precedence operator, OR ends up at the top of the tree and the tighter AND groups sit beneath it - exactly the structure you want when the executor walks it. This layering is the core trick of recursive-descent parsing; the same shape scales to as many precedence levels as a language needs.

Make it work

// layered rules encode precedence:
//   parseOr  -> parseAnd ( OR parseAnd )*
//   parseAnd -> parseCompare ( AND parseCompare )*
// the lowest-precedence operator sits at the top of the tree

CheckpointDONE

The parser respects AND-before-OR precedence when building boolean expressions. Commit and stop here.