megaparsec-5.0.0: Monadic parser combinators

Copyright © 2015–2016 Megaparsec contributors © 2007 Paolo Martini © 1999–2001 Daan Leijen FreeBSD Mark Karpov experimental non-portable None Haskell2010

Text.Megaparsec.Expr

Description

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

Synopsis

# Documentation

data Operator m a Source

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

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 `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 you don't want to allow repeating prefix or postfix operators.

`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)```