content: -- |
  -- Copyright   :  © 2015–2018 Megaparsec contributors
  --                © 2007 Paolo Martini
  --                © 1999–2001 Daan Leijen
  --
  --
  -- Code adapted from from megaparsec under the BSD license.
  module Test.Tasty.Patterns.Expr
    ( Operator (..)
    , makeExprParser )
  where
  
  import Control.Monad
  
  choice :: MonadPlus m => [m a] -> m a
  choice = msum
  
  option :: MonadPlus m => a -> m a -> m a
  option x p = p `mplus` return x
  
  -- | This data type specifies operators that work on values of type @a@. An
  -- operator is either binary infix or unary prefix or postfix. A binary
  -- operator has also an associated associativity.
  
  data Operator m a
    = InfixN  (m (a -> a -> a)) -- ^ Non-associative infix
    | InfixL  (m (a -> a -> a)) -- ^ Left-associative infix
    | InfixR  (m (a -> a -> a)) -- ^ Right-associative infix
    | Prefix  (m (a -> a))      -- ^ Prefix
    | Postfix (m (a -> a))      -- ^ Postfix
    | TernR   (m (m (a -> a -> a -> a)))
      -- ^ Right-associative ternary. Right-associative means that
      -- @a ? b : d ? e : f@ parsed as
      -- @a ? b : (d ? e : f)@ and not as @(a ? b : d) ? e : f@.
  
  -- | @'makeExprParser' term table@ builds an expression parser for terms
  -- @term@ with operators from @table@, taking the associativity and
  -- precedence specified in the @table@ into account.
  --
  -- @table@ is a list of @[Operator m a]@ lists. The list is ordered in
  -- descending precedence. All operators in one list have the same precedence
  -- (but may have different associativity).
  --
  -- Prefix and postfix operators of the same precedence associate to the left
  -- (i.e. if @++@ is postfix increment, than @-2++@ equals @-1@, not @-3@).
  --
  -- Unary operators of the same precedence can only occur once (i.e. @--2@ is
  -- not allowed if @-@ is prefix negate). If you need to parse several prefix
  -- or postfix operators in a row, (like C pointers—@**i@) you can use this
  -- approach:
  --
  -- > manyUnaryOp = foldr1 (.) <$> some singleUnaryOp
  --
  -- This is not done by default because in some cases allowing repeating
  -- prefix or postfix operators is not desirable.
  --
  -- If you want to have an operator that is a prefix of another operator in
  -- the table, use the following (or similar) wrapper instead of plain
  -- 'Text.Megaparsec.Char.Lexer.symbol':
  --
  -- > op n = (lexeme . try) (string n <* notFollowedBy punctuationChar)
  --
  -- 'makeExprParser' takes care of all the complexity involved in building an
  -- expression parser. Here is an example of an expression parser that
  -- handles prefix signs, postfix increment and basic arithmetic:
  --
  -- > expr = makeExprParser term table <?> "expression"
  -- >
  -- > term = parens expr <|> integer <?> "term"
  -- >
  -- > table = [ [ prefix  "-"  negate
  -- >           , prefix  "+"  id ]
  -- >         , [ postfix "++" (+1) ]
  -- >         , [ binary  "*"  (*)
  -- >           , binary  "/"  div  ]
  -- >         , [ binary  "+"  (+)
  -- >           , binary  "-"  (-)  ] ]
  -- >
  -- > binary  name f = InfixL  (f <$ symbol name)
  -- > prefix  name f = Prefix  (f <$ symbol name)
  -- > postfix name f = Postfix (f <$ symbol name)
  
  makeExprParser :: MonadPlus m
    => m a               -- ^ Term parser
    -> [[Operator m a]]  -- ^ Operator table, see 'Operator'
    -> m a               -- ^ Resulting expression parser
  makeExprParser = foldl addPrecLevel
  {-# INLINEABLE makeExprParser #-}
  
  -- | @addPrecLevel p ops@ adds the ability to parse operators in table @ops@
  -- to parser @p@.
  
  addPrecLevel :: MonadPlus m => m a -> [Operator m a] -> m a
  addPrecLevel term ops =
    term' >>= \x -> choice [ras' x, las' x, nas' x, tern' x, return x]
    where (ras, las, nas, prefix, postfix, tern) = foldr splitOp ([],[],[],[],[],[]) ops
          term' = pTerm (choice prefix) term (choice postfix)
          ras'  = pInfixR (choice ras) term'
          las'  = pInfixL (choice las) term'
          nas'  = pInfixN (choice nas) term'
          tern' = pTernR   (choice tern) term'
  
  -- | @pTerm prefix term postfix@ parses a @term@ surrounded by optional
  -- prefix and postfix unary operators. Parsers @prefix@ and @postfix@ are
  -- allowed to fail, in this case 'id' is used.
  
  pTerm :: MonadPlus m => m (a -> a) -> m a -> m (a -> a) -> m a
  pTerm prefix term postfix = do
    pre  <- option id prefix
    x    <- term
    post <- option id postfix
    return . post . pre $ x
  
  -- | @pInfixN op p x@ parses non-associative infix operator @op@, then term
  -- with parser @p@, then returns result of the operator application on @x@
  -- and the term.
  
  pInfixN :: MonadPlus m => m (a -> a -> a) -> m a -> a -> m a
  pInfixN op p x = do
    f <- op
    y <- p
    return $ f x y
  
  -- | @pInfixL op p x@ parses left-associative infix operator @op@, then term
  -- with parser @p@, then returns result of the operator application on @x@
  -- and the term.
  
  pInfixL :: MonadPlus m => m (a -> a -> a) -> m a -> a -> m a
  pInfixL op p x = do
    f <- op
    y <- p
    let r = f x y
    pInfixL op p r `mplus` return r
  
  -- | @pInfixR op p x@ parses right-associative infix operator @op@, then
  -- term with parser @p@, then returns result of the operator application on
  -- @x@ and the term.
  
  pInfixR :: MonadPlus m => m (a -> a -> a) -> m a -> a -> m a
  pInfixR op p x = do
    f <- op
    y <- p >>= \r -> pInfixR op p r `mplus` return r
    return $ f x y
  
  -- | Parse the first separator of a ternary operator
  
  pTernR :: MonadPlus m => m (m (a -> a -> a -> a)) -> m a -> a -> m a
  pTernR sep1 p x = do
    sep2 <- sep1
    y <- p >>= \r -> pTernR sep1 p r `mplus` return r
    f <- sep2
    z <- p >>= \r -> pTernR sep1 p r `mplus` return r
    return $ f x y z
  
  type Batch m a =
    ( [m (a -> a -> a)]
    , [m (a -> a -> a)]
    , [m (a -> a -> a)]
    , [m (a -> a)]
    , [m (a -> a)]
    , [m (m (a -> a -> a -> a))]
    )
  
  -- | A helper to separate various operators (binary, unary, and according to
  -- associativity) and return them in a tuple.
  
  splitOp :: Operator m a -> Batch m a -> Batch m a
  splitOp (InfixR  op) (r, l, n, pre, post, tern) = (op:r, l, n, pre, post, tern)
  splitOp (InfixL  op) (r, l, n, pre, post, tern) = (r, op:l, n, pre, post, tern)
  splitOp (InfixN  op) (r, l, n, pre, post, tern) = (r, l, op:n, pre, post, tern)
  splitOp (Prefix  op) (r, l, n, pre, post, tern) = (r, l, n, op:pre, post, tern)
  splitOp (Postfix op) (r, l, n, pre, post, tern) = (r, l, n, pre, op:post, tern)
  splitOp (TernR   op) (r, l, n, pre, post, tern) = (r, l, n, pre, post, op:tern)
  