Text.Parser.Expression

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

Documentation

data Assoc

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

Constructors

AssocNone

AssocLeft

AssocRight

Instances

Bounded Assoc

Defined in Text.Parser.Expression

Methods

minBound :: Assoc

maxBound :: Assoc

Enum Assoc

Defined in Text.Parser.Expression

Methods

succ :: Assoc -> Assoc

pred :: Assoc -> Assoc

toEnum :: Int -> Assoc

fromEnum :: Assoc -> Int

enumFrom :: Assoc -> [Assoc]

enumFromThen :: Assoc -> Assoc -> [Assoc]

enumFromTo :: Assoc -> Assoc -> [Assoc]

enumFromThenTo :: Assoc -> Assoc -> Assoc -> [Assoc]

Eq Assoc

Defined in Text.Parser.Expression

Methods

(==) :: Assoc -> Assoc -> Bool

(/=) :: Assoc -> Assoc -> Bool

Data Assoc

Defined in Text.Parser.Expression

Methods

gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Assoc -> c Assoc

gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c Assoc

toConstr :: Assoc -> Constr

dataTypeOf :: Assoc -> DataType

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c Assoc)

dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c Assoc)

gmapT :: (forall b. Data b => b -> b) -> Assoc -> Assoc

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Assoc -> r

gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Assoc -> r

gmapQ :: (forall d. Data d => d -> u) -> Assoc -> [u]

gmapQi :: Int -> (forall d. Data d => d -> u) -> Assoc -> u

gmapM :: Monad m => (forall d. Data d => d -> m d) -> Assoc -> m Assoc

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Assoc -> m Assoc

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Assoc -> m Assoc

Ord Assoc

Defined in Text.Parser.Expression

Methods

compare :: Assoc -> Assoc -> Ordering

(<) :: Assoc -> Assoc -> Bool

(<=) :: Assoc -> Assoc -> Bool

(>) :: Assoc -> Assoc -> Bool

(>=) :: Assoc -> Assoc -> Bool

max :: Assoc -> Assoc -> Assoc

min :: Assoc -> Assoc -> Assoc

Read Assoc

Defined in Text.Parser.Expression

Methods

readsPrec :: Int -> ReadS Assoc

readList :: ReadS [Assoc]

readPrec :: ReadPrec Assoc

readListPrec :: ReadPrec [Assoc]

Show Assoc

Defined in Text.Parser.Expression

Methods

showsPrec :: Int -> Assoc -> ShowS

show :: Assoc -> String

showList :: [Assoc] -> ShowS

Ix Assoc

Defined in Text.Parser.Expression

Methods

range :: (Assoc, Assoc) -> [Assoc]

index :: (Assoc, Assoc) -> Assoc -> Int

unsafeIndex :: (Assoc, Assoc) -> Assoc -> Int

inRange :: (Assoc, Assoc) -> Assoc -> Bool

rangeSize :: (Assoc, Assoc) -> Int

unsafeRangeSize :: (Assoc, Assoc) -> Int

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

Infix (m (a -> a -> a)) Assoc

Prefix (m (a -> a))

Postfix (m (a -> a))

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

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 :: forall m a. (Parsing m, Applicative m) => OperatorTable m a -> m a -> m a

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.

 import Control.Applicative ((<|>))
 import Text.Parser.Combinators ((<?>))
 import Text.Parser.Expression
 import Text.Parser.Token (TokenParsing, natural, parens, reserve)
 import Text.Parser.Token.Style (emptyOps)

 expr   :: (Monad m, TokenParsing m) => m Integer
 expr    = buildExpressionParser table term
         <?> "expression"

 term   :: (Monad m, TokenParsing m) => m Integer
 term    =  parens expr
         <|> natural
         <?> "simple expression"

 table  :: (Monad m, TokenParsing m) => [[Operator m Integer]]
 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)

 reservedOp name = reserve emptyOps name