megaparsec-5.3.1: Monadic parser combinators

Copyright | © 2015–2017 Megaparsec contributors © 2007 Paolo Martini © 1999–2001 Daan Leijen
License | FreeBSD
Maintainer | Mark Karpov <markkarpov92@gmail.com>
Stability | experimental
Portability | non-portable
Safe Haskell | None
Language | Haskell2010

Text.Megaparsec.Expr

Description

A helper module to parse expressions. It can build a parser given a table of operators.

Synopsis

- data Operator m a
  - = InfixN (m (a -> a -> a))
  - | InfixL (m (a -> a -> a))
  - | InfixR (m (a -> a -> a))
  - | Prefix (m (a -> a))
  - | Postfix (m (a -> a))
- makeExprParser :: MonadParsec e s m => m a -> [[Operator m a]] -> m a

# Documentation

data Operator m a #

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.

Constructors

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

makeExprParser #

Arguments

:: MonadParsec e s m |
=> m a | Term parser
-> [[Operator m a]] | Operator table, see `Operator`
-> m a | Resulting expression parser

`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 `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)