https://medium.com/@riccardoodone/a-useless-compiler-in-purescript-c0c9e8ff953e

Parser.purs

data Ast
  = Node Op Ast Ast
  | Value Int
instance showAst :: Show Ast where
  show (Node op ast1 ast2) = "(" <> show op <> " " <> show ast1 <> " " <> show ast2 <> ")"
  show (Value i) = show i

data Op
  = Add
  | Sub
instance showOp :: Show Op where
  show Add = "Add"
  show Sub = "Sub"

astParser :: Parser Ast
astParser = do
  skipSpaces
  op <- opParser
  skipSpaces
  ast1 <- valueParser <|> astParser
  skipSpaces
  ast2 <- valueParser <|> astParser
  pure $ Node op ast1 ast2

opParser :: Parser Op
opParser =
  addParser <|> subParser

addParser :: Parser Op
addParser = do
  _ <- string "add"
  pure Add

subParser :: Parser Op
subParser = do
  _ <- string "sub"
  pure Sub

intParser :: Parser Int
intParser = do
  ds <- many1 anyDigit
  case fromString $ fromChars ds of
    Just i -> pure i
    Nothing -> fail "I was expecting an int!"

valueParser :: Parser Ast
valueParser = do
  i <- intParser
  pure $ Value i