数式のパーサー実装例 / mutual 使用例

import ProgrammingHaskell.Chapter13.Section7

open Parser

mutual

  partial def expr : Parser Nat := do
    let t ← term
    let tail : Parser Nat := do
      let _ ← symbol "+"
      let e ← expr
      return t + e
    tail <|> return t

  partial def term : Parser Nat := do
    let f ← factor
    let tail : Parser Nat := do
      let _ ← symbol "*"
      let t ← term
      return f * t
    tail <|> return f

  partial def factor : Parser Nat := do
    let first : Parser Nat := do
      let _ ← symbol "("
      let e ← expr
      let _ ← symbol ")"
      return e
    first <|> natural

end

Read the issue's Lean snippet and the imported ProgrammingHaskell.Chapter13.Section7 module first. The issue does not identify a documentation file, test, or specific change, so confirm the intended location and expected form of the mutual parser example before work begins.