/Haskell 8


Copyright (c) Daan Leijen 1999-2001 (c) Paolo Martini 2007
License BSD-style (see the LICENSE file)
Maintainer [email protected]
Stability provisional
Portability non-portable
Safe Haskell Safe
Language Haskell2010


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

data Assoc Source

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

data Operator s u 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.


Infix (ParsecT s u m (a -> a -> a)) Assoc
Prefix (ParsecT s u m (a -> a))
Postfix (ParsecT s u m (a -> a))

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

An OperatorTable s u m a is a list of Operator s u 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 :: Stream s m t => OperatorTable s u m a -> ParsecT s u m a -> ParsecT s u m a Source

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 (do{ reservedOp name; return fun }) assoc
 prefix  name fun       = Prefix (do{ reservedOp name; return fun })
 postfix name fun       = Postfix (do{ reservedOp name; return fun })

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Licensed under a BSD-style license (see top of the page).