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% Earley's internals % Olle Fredriksson % Late 2015

This is an attempt to document the internals of the Earley library. The writing of this document was spurred both by a request to provide this documentation to help contributors and by some recent discussion regarding the readability of the implementation.

I'll first start with some background that many of you probably already know about. Feel free to skip ahead or skim.

Background

Let's start by having a look at how parser combinator libraries usually work. I'm aware that this doesn't give the full picture in terms of available libraries for parsing --- it ignores e.g. parser generators --- but I believe this way actually gives the best intuition for how the Earley library works. On the surface I think that the Earley library is more similar to combinator libraries than to parser generators because of the (Applicative, embedded in a powerful host language) interface that it provides. However, the functionality that it provides may more accurately be compared to parser generator libraries.

Parser combinators

I know of three ways that parser combinator libraries handle alternatives:

  • Greedy, or LL(1): Commit to an alternative the instant it has consumed a symbol. Always start with the leftmost alternative. This is classical recursive descent.
  • Backtracking: Go back and try the other alternatives if an alternative fails (i.e. depth-first).
  • Parallel: Try all alternatives in parallel (i.e. breadth-first)

Some examples of libraries in these categories are

  • Parsec, which is greedy by default, but backtracks from wherever the try combinator is used,
  • Attoparsec, which is backtracking, and
  • Parsek (a.k.a. parallel parsing processes), which is parallel.

The following is a small expression language given in BNF:

ident ::= 'x' | 'y'
expr  ::= ident | ident '+' expr

An example string in this language is "x+y". Let's investigate how the three different kinds of alternative handling cope with this input string.

  • Greedy:

    This starts by choosing the first expr branch and parsing an 'x', i.e. the derivation

    expr ---->{1} ident
         ---->{1} 'x'
         -->{'x'} EMPTY
         -------> FAIL
    

    I use the following kinds of transitions in the derivation:

    • a rest ---->{i} a_i rest: expand the leftmost non-terminal's ith production.
    • 'x' rest -->{'x'} rest: consume token 'x' from the input.

    Since the parser has managed to parse a symbol at this point, it has committed to the first expr branch and makes no further progress. Here I use EMPTY to denote the empty production (sometimes also called epsilon).

  • Backtracking:

    We take the transition FAIL ->{back} y to mean backtracking to some previous state. I'll use indentation to indicate what the state is; this is slightly informal (in an implementation we would have to save some piece of state), but hopefully clear enough.

    expr ---->{1} ident
         ---->{1} 'x'
         ------> FAIL
         ->{back}
    expr ---->{2} ident '+' expr
         ---->{1} 'x' '+' expr 
         -->{'x'} '+' expr
         -->{'+'} expr
         ---->{1} ident ---->{1} 'x'
                        -------> FAIL
                        ->{back}
                  ident ---->{2} 'y'
                        -->{'y'} EMPTY
                        -->{EOF} SUCCESS
    
  • Parallel:

    Here we use the syntax a rest ---->{*} (a_1 rest | ... | a_k rest) for expanding all the productions of a non-terminal.

    expr ---->{*} ident | ident '+' expr
         ---->{*} 'x' | 'y' | 'x' '+' expr | 'y' '+' expr
         -->{'x'} EMPTY | '+' expr
         -->{'+'} expr
         ---->{*} ident | ident '+' expr
         ---->{*} 'x' | 'y' | 'x' '+' expr | 'y' '+' expr
         -->{'y'} EMPTY | '+' expr
         -->{EOF} SUCCESS
    

We saw that greedy parsing did not succeed like the others. So what is it good for? First of all, we can rewrite the grammar and make it work. For example, as follows:

ident ::= 'x' | 'y'
tail ::= '+' ident tail | EMPTY
expr  ::= ident tail

For the example string, "x+y", we get the following derivation:

expr ---->{1} ident tail
     ---->{1} 'x' tail
     -->{'x'} tail
     ---->{1} '+' ident tail
     -->{'+'} ident tail ---->{1} 'x' tail
                         -------> FAIL
                         ->{back}
              ident tail ---->{2} 'y' tail
                         -->{'y'} tail ---->{1} '+' ident tail
                                       -------> FAIL
                                       ->{back}
                                  tail ---->{2} EMPTY
                                       -------> SUCCESS

Note that this parser also backtracks, but only if it hasn't consumed any input in the branch it backtracks from. The nice property of a greedy parser, compared to a backtracking parser, is that it never backtracks in the input string. This means that it is potentially more efficient than a backtracking parser, because it does not have to keep references to old positions in the input string when keeping backtracking states. I say potentially faster, because Attoparsec, probably the fastest parser combinator library for Haskell currently in existence, does full backtracking seemingly without suffering from any performance penalty.

Both backtracking and parallel parsing have the same power in terms of the languages that they recognise. If we are only interested in recognition and our language is not LL(1) or not easily factorable into LL(1), backtracking parsing is a practical choice, since it often does less work than a parallel parser. Note, however, that there are cases where a backtracking parser has to do as much work as a parallel parser. If we are interested in getting all the ways a given string is parsable, parallel parsing is a natural choice.

The limitations of parser combinators

  • Language:

    Backtracking and parallel parsers are guaranteed to terminate only for any LL(k) language. For example, they do not handle left-recursive grammars, i.e. grammars where we have derivations of the form p -->+ p rest.

    As an example, they cannot handle the following grammar, because expr is left-recursive:

    ident ::= 'x' | 'y'
    expr  ::= ident | expr '+' ident
    

    In practice, we can often perform left-recursion removal. The cost of this is that the intended meaning of the grammar is sometimes obscured.

  • Worst-case running time:

    There are many grammars for which parsing is exponential.

    Consider:

    x ::= '(' x x ')' | EMPTY
    

    Let's see what happens in trying to parse the string "(())" in parallel:

    x ---->{*} '(' x x ')' | EMPTY
      -->{'('} x x ')'
      ---->{*} '(' x x ')' x ')' | x ')'
      ---->{*} '(' x x ')' x ')' | '(' x x ')' ')' | ')'
      -->{'('} x x ')' x ')' | x x ')' ')'
      ---->{*} '(' x x ')' x x ')' x ')' | x ')' x ')' | x x ')' ')'
      ---->{*} '(' x x ')' x x ')' x ')' | '(' x x ')' ')' x ')' | ')' x ')' | x x ')' ')'
      ---->{*} '(' x x ')' x x ')' x ')' | '(' x x ')' ')' x ')' | ')' x ')' | '(' x x ')' x ')' ')' | x ')' ')'
      ---->{*} '(' x x ')' x x ')' x ')' | '(' x x ')' ')' x ')' | ')' x ')' | '(' x x ')' x ')' ')' | '(' x x ')' ')' ')' | ')' ')'
      -->{')'} x ')' | ')'
      ---->{*} '(' x x ')' ')' | ')' | ')'
      -->{')'} EMPTY | EMPTY
      -------> SUCCESS
    

    We can at least see that the number of parallel productions that we are processing blows up quite quickly. I find it easiest to see this problem in the parallel strategy, but note that backtracking parsing has to do the same amount of work as the parallel parser for certain inputs.

    This should also have convinced you that quite a lot of the work is duplicated.

Both of these problems stem from the way that parser combinator grammars are written. Most recursive grammars written using parser combinators are not finite, even though they have a finite representation, and there is no way to detect this.

This is similar to how we can't detect (without using dirty, unsafe tricks) that we will never reach the end of the list defined by ones = 1 : ones.

In this light, let's look at the left-recursive grammar again:

expr ::= ident | expr '+' ident

As a parser combinator grammar, this is equivalent to the following, which elucidates the left-recursion problem:

expr ::= ident | (ident | (ident | (ident | ...) '+' ident) '+' ident) '+' ident

Since parser combinators have no way of distinguishing between non-terminals and their productions, they also have no way of sharing work between invocations of the same non-terminal, because they cannot detect that this has happened in the first place.

How not to repeat yourself yourself

I know of two situations where there is potential to share work.

The first is when we encounter the same non-terminal at the same position. As an example, there is no need to re-do the work of x in both branches of a in the following grammar.

a ::= x rest1 | x rest2

A better idea would be to just parse x once and carry on with rest1 | rest2. Pictorially, we want the following branching structure:

       rest1
      /
 a---x
      \
       rest2

We do not want the following, which we have in parallel parsing:

   x--rest1
  /
 a
  \
   x--rest2

A more general example is whenever we are parsing two unrelated branches at the same position, such as:

a ::= start1 x rest1
b ::= start2 x rest2

If we are parsing a and b at the same time we also want to share the work of the non-terminal x whenever it's encountered at the same position:

a---start1   --rest1
          \ /
           x
          / \
b---start2   --rest2

What we don't want is to keep the two branches independent:

a---start1---x---rest1

b---start2---x---rest2

It turns out that if we never re-do the work of a non-terminal, we can automatically gain support for left-recursive grammars, since the main problem there is the infinite expansion of recursive non-terminals.

The second way we can share work is perhaps less obvious. Here I will use an extended BNF syntax where productions can have alternatives anywhere in the tree. As an example

a ::= (x | y) rest

means that a accepts either an x or a y, followed by rest. One (bad) way to handle (x | y) rest is to desugar it as follows:

(x | y) rest = x rest | y rest

Now let's say that in parsing a, both branches x and y are successful and finish at the same time. Unless rest is a non-terminal and falls into the work-sharing situation above, we will be parsing two instances of the same thing. It would be more efficient if we could share the work of rest.

Pictorially, we want the following branching structure:

  x--
 /   \
a     rest
 \   /
  y--

We do not want the following, which we sometimes have in combinator libraries.

  x---rest
 /
a
 \
  y---rest

This second point also applies if instead of x | y we have a non-terminal that expands to multiple branches.

The Earley library and the essence of Earley

There are already good presentations of Earley's algorithm available, so I will not repeat the full definition here. To give some context, here's the core of the algorithm, adapted from Wikipedia:


The state set at input position k is called S(k). The parser is seeded with S(0) consisting of only the top-level rule. The parser then repeatedly executes three operations: prediction, scanning, and completion.

  • Prediction: For every state in S(k) of the form (X -> a . Y b, j) (where j is the origin position as above), add (Y -> . y, k) to S(k) for every production in the grammar with Y on the left-hand side (Y -> y).
  • Scanning: If a is the next symbol in the input stream, for every state in S(k) of the form (X -> a . 'a' b, j), add (X -> a 'a' . b, j) to S(k+1).
  • Completion: For every state in S(k) of the form (X -> y ., j), find states in S(j) of the form (Y -> a . X b, i) and add (Y -> a X . b, i) to S(k).

It is important to note that duplicate states are not added to the state set, only new ones. These three operations are repeated until no new states can be added to the set. The set is generally implemented as a queue of states to process, with the operation to be performed depending on what kind of state it is.


The note that duplicate states are not added is very important. It means that:

  1. If the prediction step expands the same non-terminal multiple times at the same position, no more than one copy of each of the non-terminal's productions are added to the current state set.

  2. If the completion step completes a non-terminal multiple times at the same position, no more than one copy of each of the completions from the earlier state that it refers to are added to the current state set.

These points are the essence of Earley parsing, and mean that we are work-sharing precisely as outlined above.

State sets

The state set does not have to be a set, as long as we follow the two points above (note that they make no use of set-specific properties, but only that we do not expand the same thing more than once per position). We can use whatever representation of state collections that we find appropriate as long as we do that. In the Earley library, productions can contain functions, e.g. of type token -> Bool for matching input tokens with arbitrary predicates, which means that they are not in general comparable (they are not in the Ord typeclass). Since this means that we cannot use Set or similar containers, we instead use lists of states.

Additionally, we do not have to keep states from earlier positions around, as long as we have enough information to perform the completion step.

The Earley library keeps only two lists of Earley states: One for the current position and one for the next position, and follows the two points above by machinery that will now be described.

The representation of states is different from the classical presentations of Earley's algorithm. We represent a state as a production and a continuation pointer. We keep only the part of the production that is left to parse, i.e. we drop everything before "the dot" in Earley's states. We have a special Final state for when we are done. The continuation pointer is a mutable reference to a list of continuations, which correspond to possible completions in the original algorithm. For the moment we can think of a continuation as being the same kind of thing as a state, though technically they differ slightly because when we are also dealing with parse results, continuations accept such a parse result before they can meaningfully be followed.

Let's have a look at an example. A state (a production and a mutable reference to a list of states) might look a little bit like this:

  +------------------+
  |                  |
  v                  |
cont1       (prod1, ptr)
cont2

I said that continuations are pretty much also states, so there might be more continuation pointers going left until we reach the special Final state.

The parsing operations

When parsing we have a list of such states for the current position, and a list for the next position. When all the states for the current position have been processed, we advance the position in the input string and make the next-states the current, and use the empty list for the new next-states.

The scanning step is done just like in Earley's original algorithm; when processing (prod1, ptr), if prod1 = 'x' prod1' and the current position in the input is an 'x', then we add (prod1', ptr) to the list for the next position.

The prediction step, i.e. the expansion of a non-terminal, is a bit more peculiar. First we need to look at how a non-terminal is represented.

We are using the generalised representation of productions from above where alternatives can occur anywhere in the tree (which also means that productions are in the standard Haskell Alternative typeclass), so we just need to associate a production (and not e.g. a list of productions) with every non-terminal.

Every non-terminal is also associated with a mutable reference to a mutable reference (no, that is not a typo) to a list of continuations. So a non-terminal might look a bit like this:

      cont1
      cont2
      cont3
       ^
   +---+
   |
   |
innerPtr<--+
           |
           |
(prod, outerPtr)

The outermost reference (outerPtr above) is made sure to point to a fresh innerPtr that points to the empty list every time we advance the position in the input string.

Since both the production and the pointer depend on the type of the non-terminal, we cannot easily store this information in e.g. a Map because it would have to contain elements of different types (i.e. be heterogeneous).

The solution to this typed associated-information problem, perhaps obvious to some, is to store the associated information in the non-terminals. The representation of non-terminals in the library is thus exactly this associated information: a production and the reference reference.

Now we are ready to do prediction. Let's say our current-list is [(x prod1, ptr1), (x prod2, ptr2)], where the non-terminal x = (xProd, xPtr), and our next-list is []. This means that we have two states that begin with the same non-terminal, but end with different productions (prod1 and prod2), and that additionally have continuations from before (ptr1 and ptr2).

Assuming that it hasn't been expanded before at this position, x looks something like this:

      [ ]
       ^
   +---+
   |
   |
innerXPtr<--+
            |
            |
  (xProd, xPtr)

The prediction step takes place when we process our first state, (x prod1, ptr1). We then add the continuation (prod1, ptr1) to the inner list that x refers to, and add the state (xProd, innerXPtr) to the current-list.

When we process the second state (x prod2, ptr2), we can detect that x has already been expanded at this position, because the continuation list is non-empty. When this happens we add (prod2, ptr2) to the inner list that x refers to, but we do not add any new state to the current-list --- because xProd is already there. At this point we have the following pointer structure:

 (prod1, ptr1)
 (prod2, ptr2)         current-list
       ^            (xProd, innerXPtr)
       |                        |
   +---+------------------------+
   |
innerXPtr<--+
            |
            |
  (xProd, xPtr)

We can see in the above that we are sharing the work of parsing x, and simply keep track of the two continuations for when that has been done. Here we can also see why we need a double reference from non-terminals. We clear every non-terminal when we advance the position, but we still want to keep the pointers between any states and continuations. Clearing in the above picture would mean to mutate xPtr to point to a fresh inner reference that points to the empty list. However, the innerXPtr from any states in the next-list stay intact.

Now we come to completion, which is what happens when we process a state of the form (EMPTY, ptr). Then we simply look up the continuations by following ptr and add those to the current-list. Pretending that xProd was EMPTY In the above picture, we would add (prod1, ptr1) and (prod2, ptr2) to the current-list.

Since we do not tie alternatives to non-terminals, we have an additional operation which happens when we encounter alternatives. In the original Earley algorithm this is baked into the prediction step, and in our formulation of the algorithm this operation is basically the same as prediction.

When processing a state ((alt1 | alt2) prod, ptr) we create a new continuation pointer, newptr that points to (prod, ptr), and continue with the states (alt1, newptr) and (alt2, newptr). With this operation in place we have to be more careful when we do completion. Just like we made sure that a non-terminal is only expanded once per position in the input we have to make sure that completion of a continuation is only done once per position. For now we can think of this as pairing a mutable boolean with each list of continuations that we point to, though this is slighly more complicated when we also have to deal with parse results.

TODO simplifyCont/Leo's optimisation

Recognition vs. parsing

A recogniser for a language is a program that decides if a given input string is in the language. Parsing additionally creates a parse tree. So far we have only really discussed recognition.

In Haskell it is convenient to give an Applicative interface to our parsers, which allows us to attach semantic actions to parsers without having to construct and interpret an intermediate parse tree.

TODO explain delayed results and how they cope with infinitely and exponentially many result

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