- The Classes Defining the Interface
- The type describing parsers:
- Additional useful combinators
- Controlling the text of error reporting:
- An alternative for the Alternative, which is greedy:
- Parsers can be disambiguated using micro-steps:
- Dealing with (non-empty) Ambigous parsers:
- Parse errors can be retreived from the state:
- Starting and finalising the parsing process:
- The state may be temporarily change type:
- A more efficient version for
from the module
- Controlling the text of error reporting:
- Maintaining Progress Information
- Auxiliary functions and types
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
- class (Applicative p, ExtApplicative p, Alternative p) => IsParser p
- class ExtApplicative p where
- (<$) :: a -> p b -> p a
- class Symbol p symbol token | p symbol -> token where
- class Provides state symbol token | state symbol -> token where
- class Eof state where
- data P st a = P (forall r. (a -> st -> Steps r) -> st -> Steps r) (forall r. (st -> Steps r) -> st -> Steps (a, r)) (forall r. (st -> Steps r) -> st -> Steps r) Nat (Maybe a)
- (<?>) :: P state a -> String -> P state a
- amb :: P st a -> P st [a]
- class Stores state error | state -> error where
- getErrors :: state -> ([error], state)
- pErrors :: Stores st error => P st [error]
- pEnd :: (Stores st error, Eof st) => P st [error]
- parse :: Eof t => P t a -> t -> a
- pSwitch :: (st1 -> (st2, st2 -> st1)) -> P st2 a -> P st1 a
- type Cost = Int
- type Progress = Int
- type Strings = [String]
- data Steps a where
- eval :: Steps a -> a
- push :: v -> Steps r -> Steps (v, r)
- apply :: Steps (b -> a, (b, r)) -> Steps (a, r)
- pushapply :: (b -> a) -> Steps (b, r) -> Steps (a, r)
- norm :: Steps a -> Steps a
- best :: Steps a -> Steps a -> Steps a
- best' :: Steps b -> Steps b -> Steps b
- getCheapest :: Int -> [(Int, Steps a)] -> Steps a
- traverse :: Int -> Steps a -> Int -> Int -> Int
- removeEnd_h :: Steps (a, r) -> Steps r
- removeEnd_f :: Steps r -> Steps [r]
- must_be_non_empty :: [Char] -> P t t1 -> t2 -> t2
- must_be_non_empties :: [Char] -> P t1 t -> P t3 t2 -> t4 -> t4
- data Nat
- module Control.Applicative
The Classes Defining the Interface
This class collects a number of classes which together defines what a
Parser should provide.
Since it is just a predicate we have prefixed the name by the phrase
The module Control.Applicative contains the definition for
<$ 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.
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.
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
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
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`
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.
The type describing parsers:
|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)|
|Monad (P st)|
|Functor (P state)|
|MonadPlus (P st)|
|Applicative (P state)|
|Alternative (P state)|
|ExtApplicative (P st)|
In the new module
|Provides state symbol token => Symbol (P state) symbol token|
Parsers are functors:
Additional useful combinators
Controlling the text of error reporting:
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.
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:
Dealing with (non-empty) Ambigous parsers:
Parse errors can be retreived from the state:
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.
pEnd should be called at the end of the parsing process. It deletes any unsonsumed input, and reports its preence as an eror.
The state may be temporarily change type:
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
Maintaining Progress Information
The data type
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
which tells how much of the input state was consumed.
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
constructors is as follows:
- A token was succesfully recognised, and as a result the input was
advancedby the distance
- The type of value represented by the
Stepschanges by applying the function parameter.
- 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
- 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..
Auxiliary functions and types
Checking for non-sensical combinations:
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
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.
for describing the minimal number of tokens consumed
The data type
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-add` more than necesssary.