expression_parser.pest

// Grammar demonstrating patterns for:
// 1. Repetition of anonymous sequences: comparison = { arith_expr ~ (comp_op ~ arith_expr)* }
// 2. Nested choices in repetitions: arith_expr = { term ~ ((plus|minus) ~ term)* }
// 3. Parsing into enums for operators

WHITESPACE = _{ " " | "\t" | "\n" | "\r" }

// Literals - using @ (atomic) to prevent WHITESPACE from being included in spans
number = @{ ASCII_DIGIT+ }
identifier = @{ ASCII_ALPHA ~ (ASCII_ALPHANUMERIC | "_")* }

// Atomic expressions
atom = { number | identifier | "(" ~ expr ~ ")" }

// Multiplicative operators (for nested choice example)
mul = { "*" }
div = { "/" }

// Additive operators (for nested choice example)
plus = { "+" }
minus = { "-" }

// Comparison operators (for nested choice example)
eq = { "==" }
neq = { "!=" }
lt = { "<" }
gt = { ">" }

// Expressions with operator precedence
// Each level demonstrates the pattern: term ~ (operator ~ term)*

// Factor: atom
factor = { atom }

// Term: factor ~ ((mul | div) ~ factor)*
// This demonstrates nested choice (mul | div) in a repetition
term = { factor ~ ((mul | div) ~ factor)* }

// Arithmetic expression: term ~ ((plus | minus) ~ term)*
// This demonstrates nested choice (plus | minus) in a repetition
arith_expr = { term ~ ((plus | minus) ~ term)* }

// Comparison: arith_expr ~ (comp_op ~ arith_expr)*
// This demonstrates using a combined operator rule for cleaner AST
comp_op = { eq | neq | lt | gt }
comparison = { arith_expr ~ (comp_op ~ arith_expr)* }

// Top-level expression
expr = { comparison }