Text.ParserCombinators.UU.Core

The module Core contains the basic functionality of the parser library. It uses the breadth-first module to realise online generation of results, the error correction administration, dealing with ambigous grammars; it defines the types of the elementary parsers and recognisers involved.For typical use cases of the libray see the module Text.ParserCombinators.UU.Examples

The Classes Defining the Interface

IsParser

class (Applicative p, ExtApplicative p, Alternative p) => IsParser p

This class collects a number of classes which together defines what a Parser should provide. Since it is just a predicate we have prefixed the name by the phrase Is

Instances

(Applicative p, ExtApplicative p, Alternative p) => IsParser p

ExtApplicative

class ExtApplicative p where

The module Control.Applicative contains the definition for <$ which cannot be changed . Since we want to give optimised implementations of this combinator, we hide its definition, and define a class containing its signature.

Methods

(<$) :: a -> p b -> p a

Instances

ExtApplicative (P st)

In the new module Control.Applicative the operator <$ is still hard coded. We hide this import and provide an implementation using a class, which can be redfined when needed.

Symbol

class Symbol p symbol token | p symbol -> token where

Many parsing libraries do not make a distinction between the terminal symbols of the language recognised and the tokens actually constructed from the input. This happens e.g. if we want to recognise an integer or an identifier: we are also interested in which integer occurred in the input, or which identifier.

The function pSym takes as argument a value of some type symbol, and returns a value of type token. The parser will in general depend on some state which is maintained holding the input. The functional dependency fixes the token type, based on the symbol type and the type of the parser p. Since pSym is overloaded both the type and the value of symbol determine how to decompose the input in a token and the remaining input. pSymExt is the actual function, which takes two extra parameters: one describing the minimal numer of tokens recognised, and the second whether the symbol can recognise the empty string and the value which is to be returned in that case

Methods

pSymExt :: Nat -> Maybe token -> symbol -> p token

The first parameter to pSymExt is a Nat which describes the minimal numer of tokens accepted by this parser. It is used in the abstract interpretation which computes this property for each parser. It's main use is in choosinga non-recursive alternative in case a non-terminal has to be inserted. The second parameter indicates whether this parser can also skip recognising anything and just return a value of type a, hence a `Maybe a`

pSym :: symbol -> p token

Instances

Provides state symbol token => Symbol (P state) symbol token

Provides

class Provides state symbol token | state symbol -> token where

The function splitStae playes a crucial role in splitting up the state. The symbol parameter tells us what kind of thing, and even which value of that kind, is expected from the input. The state and and the symbol type together determine what kind of token has to be returned. Since the function is overloaded we do not have to invent all kind of different names for our elementary parsers.

Methods

splitState :: symbol -> (token -> state -> Steps a) -> state -> Steps a

Instances

(Eq a, Show a) => Provides (Str a) a a

(Show a, Eq a) => Provides (Str a) (Token a) [a]

Show a => Provides (Str a) (Munch a) [a]

(Ord a, Show a) => Provides (Str a) (a, a) a

Show a => Provides (Str a) (a -> Bool, String, a) a

Eof

class Eof state where

Methods

eof :: state -> Bool

deleteAtEnd :: state -> Maybe (Cost, state)

Instances

Show a => Eof (Str a)

The type describing parsers: P

data P st a

Constructors

P (forall r. (a -> st -> Steps r) -> st -> Steps r) (forall r. (st -> Steps r) -> st -> Steps (a, r)) (forall r. (st -> Steps r) -> st -> Steps r) Nat (Maybe a)

Instances

Monad (P st)

Functor (P state)

MonadPlus (P st)

Applicative (P state)

Alternative (P state)

ExtApplicative (P st)

In the new module Control.Applicative the operator <$ is still hard coded. We hide this import and provide an implementation using a class, which can be redfined when needed.

Provides state symbol token => Symbol (P state) symbol token

Parsers are functors: fmap

Parsers are Applicative: <*>, <*, *> and pure

Parsers are Alternative: <|> and empty

Parsers can recognise single tokens: pSym and pSymExt

Parsers are Monads: >>= and return

Additional useful combinators

Controlling the text of error reporting: <?>

(<?>) :: P state a -> String -> P state a

The parsers build a list of symbols which are expected at a specific point. This list is used to report errors. Quite often it is more informative to get e.g. the name of the non-terminal. The <?> combinator replaces this list of symbols by it's righ-hand side argument.

An alternative for the Alternative, which is greedy: <<|>

Parsers can be disambiguated using micro-steps: micro

Dealing with (non-empty) Ambigous parsers: amb

amb :: P st a -> P st [a]

Parse errors can be retreived from the state: pErrors

class Stores state error | state -> error where

getErrors retreives the correcting steps made since the last time the function was called. The result can, using a monad, be used to control how to-- proceed with the parsing process.

Methods

getErrors :: state -> ([error], state)

Instances

Stores (Str a) (Error Int)

pErrors :: Stores st error => P st [error]

Starting and finalising the parsing process: pEnd and parse

pEnd :: (Stores st error, Eof st) => P st [error]

The function pEnd should be called at the end of the parsing process. It deletes any unsonsumed input, and reports its preence as an eror.

parse :: Eof t => P t a -> t -> a

The state may be temporarily change type: pSwitch

pSwitch :: (st1 -> (st2, st2 -> st1)) -> P st2 a -> P st1 a

pSwitch takes the current state and modifies it to a different type of state to which its argument parser is applied. The second component of the result is a function which converts the remaining state of this parser back into a valuee of the original type.

A more efficient version for <$ from the module Control.Applicative

Maintaining Progress Information

type Cost = Int

The data type Steps is the core data type around which the parsers are constructed. It is a describes a tree structure of streams containing (in an interleaved way) both the online result of the parsing process, and progress information. Recognising an input token should correspond to a certain amount of Progress, which tells how much of the input state was consumed. The Progress is used to implement the breadth-first search process, in which alternatives are examined in a more-or-less synchonised way. The meaning of the various Step constructors is as follows:

Step

A token was succesfully recognised, and as a result the input was advanced by the distance Progress

Apply

The type of value represented by the Steps changes by applying the function parameter.

Fail

A correcting step has to made to the input; the first parameter contains information about what was expected in the input, and the second parameter describes the various corrected alternatives, each with an associated Cost

Micro

A small cost is inserted in the sequence, which is used to disambiguate. Use with care!

The last two alternatives play a role in recognising ambigous non-terminals. For a full description see the technical report referred to from the README file..

type Progress = Int

type Strings = [String]

data Steps a where

Constructors

Step :: Progress -> Steps a -> Steps a

Apply :: forall a b. (b -> a) -> Steps b -> Steps a

Fail :: Strings -> [Strings -> (Cost, Steps a)] -> Steps a

Micro :: Cost -> Steps a -> Steps a

End_h :: ([a], [a] -> Steps r) -> Steps (a, r) -> Steps (a, r)

End_f :: [Steps a] -> Steps a -> Steps a

eval :: Steps a -> a

push :: v -> Steps r -> Steps (v, r)

apply :: Steps (b -> a, (b, r)) -> Steps (a, r)

pushapply :: (b -> a) -> Steps (b, r) -> Steps (a, r)

norm :: Steps a -> Steps a

best :: Steps a -> Steps a -> Steps a

best' :: Steps b -> Steps b -> Steps b

getCheapest :: Int -> [(Int, Steps a)] -> Steps a

traverse :: Int -> Steps a -> Int -> Int -> Int

removeEnd_h :: Steps (a, r) -> Steps r

removeEnd_f :: Steps r -> Steps [r]

Auxiliary functions and types

Checking for non-sensical combinations: must_be_non_empty and must_be_non_empties

must_be_non_empty :: [Char] -> P t t1 -> t2 -> t2

The function checks wehther its second argument is a parser which can recognise the mety sequence. If so an error message is given using the name of the context. If not then the third argument is returned. This is useful in testing for loogical combinations. For its use see the module Text>parserCombinators.UU.Derived

must_be_non_empties :: [Char] -> P t1 t -> P t3 t2 -> t4 -> t4

This function is similar to the above, but can be used in situations where we recognise a sequence of elements separated by other elements. This does not make sense if both parsers can recognise the empty string. Your grammar is then highly ambiguous.

The type Nat for describing the minimal number of tokens consumed

data Nat

The data type Nat is used to represent the minimal length of a parser. Care should be taken in order to not evaluate the right hand side of the binary functions nat_min and `nat-add` more than necesssary.

Constructors

Zero

Succ Nat

Infinite

Instances

Show Nat

module Control.Applicative