Portability | non-portable |
---|---|

Stability | experimental |

Maintainer | ekmett@gmail.com |

Safe Haskell | Safe-Inferred |

A helper module to parse "expressions". Builds a parser given a table of operators and associativities.

- data Assoc
- = AssocNone
- | AssocLeft
- | AssocRight

- data Operator m a
- type OperatorTable m a = [[Operator m a]]
- buildExpressionParser :: (Parsing m, Monad m) => OperatorTable m a -> m a -> m a

# Documentation

This data type specifies the associativity of operators: left, right or none.

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.

type OperatorTable m a = [[Operator m a]]Source

An `OperatorTable m a`

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 a different associativity).

buildExpressionParser :: (Parsing m, Monad m) => OperatorTable m a -> m a -> m aSource

`buildExpressionParser table term`

builds an expression parser for
terms `term`

with operators from `table`

, taking the associativity
and precedence specified in `table`

into account. Prefix and postfix
operators of the same precedence can only occur once (i.e. `--2`

is
not allowed if `-`

is prefix negate). Prefix and postfix operators
of the same precedence associate to the left (i.e. if `++`

is
postfix increment, than `-2++`

equals `-1`

, not `-3`

).

The `buildExpressionParser`

takes care of all the complexity
involved in building expression parser. Here is an example of an
expression parser that handles prefix signs, postfix increment and
basic arithmetic.

expr = buildExpressionParser table term <?> "expression" term = parens expr <|> natural <?> "simple expression" table = [ [prefix "-" negate, prefix "+" id ] , [postfix "++" (+1)] , [binary "*" (*) AssocLeft, binary "/" (div) AssocLeft ] , [binary "+" (+) AssocLeft, binary "-" (-) AssocLeft ] ] binary name fun assoc = Infix (fun <* reservedOp name) assoc prefix name fun = Prefix (fun <* reservedOp name) postfix name fun = Postfix (fun <* reservedOp name)