Copyright | © 2015–2018 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 |

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

## Synopsis

- data Operator m a
- makeExprParser :: MonadParsec e s m => m a -> [[Operator m a]] -> m a

# Documentation

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.

:: MonadParsec e s m | |

=> m a | Term parser |

-> [[Operator m a]] | Operator table, see |

-> m a | Resulting expression parser |

builds an expression parser for terms
`makeExprParser`

term table`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)