📙 crustimoney

A Clojure library for PEG parsing, supporting various grammars, packrat caching and... cuts?

Motivation

The first version of crustimoney was my first library in Clojure, a long time ago. Simply put, this version is the mental exercise of making it better. I like to think it turned out well. Maybe you like it too.

"What does Crustimoney Proseedcake mean?" said Pooh. "For I am a Bear of Very Little Brain, and long words Bother me."

"It means the Thing to Do."

"As long as it means that, I don't mind," said Pooh humbly.

Let's parse those long words from Owl.

Features

• Create parsers from combinator functions
• .. or from string-based definitions
• .. or from data-driven definitions
• Packrat caching, optimizing cpu usage
• Concept of cuts, optimizing memory usage - and error messages
• Focus on capture groups, resulting in shallow parse trees
• Minimal parse tree data, fetch only what's needed on post-processing
• Virtual stack, preventing overflows
• Infinite loop detection (runtime)
• Missing rule references detection (compile time)
• Streaming support (experimental)

Add to project

The instructions for the latest version can be found here:

Quick!

In a hurry, and you just need to parse a small text where a regular expression just doesn't cut it? For this there's the crustimoney.quick/parse function. It takes a string- or data-driven parser definition plus a text. The definition can use the built-in parsers. It returns a conveniently transformed result if it matched. For example:

(quick/parse '("alice" (" and " (:who word))+)
             "alice and bob and eve")

=> [nil "alice and bob and eve"
    [:who "bob"]
    [:who "eve"]]

As you can see, the captured texts are directly availabe in the result.

If this is what you need - right now! - you could skip directly to string-based grammar or data-based grammar. For all the other details, read on!

Combinator grammar

The combinators are at the heart of the library. Even though you may never use them directly, it is a good starting point. Below is a list of available combinators, found in the crustimoney.combinator-grammar namespace.

The essentials:

• literal, match an exact literal string
• chain, chain multiple consecutive parsers
• choice, match the first successful parsers
• repeat*, eagerly match a parser as many times as possible
• negate, succeed if the given parser does not

Those are actually enough for parsing any unambiguous text. But more combinators are provided, for ease of use, nicer result trees and better performance:

• regex, match a regular expression
• repeat+, same as repeat*, but require at least one match
• lookahead, succeed if the given parser does, without advancing
• maybe, try the given parser, succeed anyway
• eof, succeed if there is no more input

Each combinator returns a parser (model). Some combinators take one or more parsers, making them composable. For example:

(chain (literal "foo") (regex "ba(r|z)"))

Such a parser can be supplied to core/parse, together with the string to parse.

Parse results

The result is a "hiccup"-style parse tree, for example:

[:node {:start 0, :end 6}
 [:child-node {:start 0, :end 3}]
 [:child-node {:start 3, :end 6}]]

To capture a node during parsing, it must be "named", such as :node or :child-node in above example. This is done by wrapping a parser with with-name (or by other means, depending on the grammar type). Results without a name are filtered out, though its named children are kept. The root node can be nameless (nil).

On failed parses, a set of errors is returned, which has the following structure:

#{{:key :expected-literal, :at 10, :detail {:literal "foo"}}
  {:key :expected-match, :at 8, :detail {:regex "alice|bob"}}
  {:key :unexpected-match, :at 8, :detail {:text "eve"}}}

If you want to override the default key of an error, a parser can be wrapped with with-error. For example:

(def parser
  (with-error :number-required
    (regex #"\d+")))

(core/parse parser "nan")
=> #{{:key :number-required, :at 0}}

To work with these successes and errors, the functions in the results namespace can be used. These allow you to get the text of a success node for example, or add :line and :column keys to the errors. It also contains tools to walk and transform the tree (see built-in transformer).

Recursive grammars

Composing a single parser can be enough in some cases. More complex texts need or are better expressed with a recursive grammar, i.e. named parsers that can refer to each other. For example:

(def my-grammar
   {:root (repeat+ (choice :foo :bax))
    :foo  (literal "foo")
    :bax  (regex "ba(r|z)")})

This grammar can be used as follows:

(core/parse my-grammar "foobaz")
=> [nil {:start 0, :end 6}]

Such a map requires a :root rule to be present.

Nested recursive grammars

An advanced feature is lexically-scoped nested recursive grammars. A contrived example of this is:

{:foo    (literal "foo")
 :bar    (literal "wrong")
 :foobar {:root (chain :foo :bar)
          :bar  (literal "bar")}
 :root   (ref :foobar)}

Inner maps can refer to rules in its own scope and the enclosing scopes. Inner rules take precedence over outer rules with the same name. Outer scopes can not refer to inner scopes.

Compiling the grammar

Note that the grammar model is compiled on-the-fly by core/parse. This will check for a :root rule and dangling references.

To do this compiling beforehand, you can use core/compile. It is recommended to do this in production code, as it speeds up consecutive parse calls considerably.

Auto-named rules

The example above shows that all success nodes are filtered out, except the root node. This is because the results were nameless. The parsers could be wrapped with with-name, but the names would probably be the same as the rule names in this case. Appending an = to the rule name will automatically wrap the parser with with-name. This would update the grammar to:

{:root= (repeat+ (choice :foo :bax))
 :foo=  (literal "foo")
 :bax=  (regex "ba(r|z)")})

Note that the refernce keys are still without the postfix. Parsing it again would yield the following result:

[:root {:start 0, :end 6}
 [:foo {:start 0, :end 3}]
 [:bax {:start 3, :end 6}]]

A word of caution though. It is encouraged to be very intentional about which nodes should be captured and when. For example, using the following grammar would only yield a :wrapped node if the :expr is really wrapped in parentheses:

(choice (with-name :wrapped
          (chain (literal "(")
                 (ref :expr)
                 (literal ")")))
        (ref :expr))

This approach results in shallower result trees and thus less post-processing.

Cuts

Most PEG parsers share a downside: they are memory hungry. This is due to their packrat caching, that provides one of their upsides: linear parsing time.

This paper describes adding cuts to PEGs, a concept that is known from Prolog. Crustimoney expands on this by differentiating between hard cuts and soft cuts.

Hard cuts

A hard cut tells the parser that it should never backtrack beyond the position where it is encountered. This has two major benefits. The first is better and more localized error messages. The following example shows this, and also how to add a hard cut in the chain combinator.

(def example
  (maybe (chain (literal "(")
                hard-cut
                (regex #"\d+")
                (literal ")"))))

(core/parse example "(42")
=> #{{:key :expected-literal, :at 3, :detail {:literal ")"}}}

Without the hard cut, the parse would be successful (because of the maybe combinator). But, since the text clearly opens a parenthesis, it would be better to fail. The hard cut enforces this, as the missing ")" error cannot backtrack beyond it. So from a user's standpoint, a cut can already be very beneficial.

The second major benefit is that the parser can release everything in its cache before the cut position. It will never need this again. This behaviour makes that well placed hard cuts can - especially when parsing repeating structures - alleviate the memory requirements to be constant.

Note that a cut can only be used within a chain, and never as the first element. The preceding elements should consume some input, and that input should only be valid for element at that point in the text.

An alias for the hard-cut is >>.

Soft cuts

There are situations that localized error messages are desired, but backtracking should still be possible. For such situations a soft cut can be used. Such a cut also disallows backtracking, but only while inside the chain. Once the chain is successfully parsed, the soft cut has no effect anymore.

Consider the expansion of the previous example:

(def example
  (choice (chain
            ;; --- same as before, but now with soft-cut
            (maybe (chain (literal "(")
                          soft-cut
                          (regex #"\d+")
                          (literal ")")))
            ;; ---
            (literal "foo"))
          (literal "bar")))

(core/parse example "(42")
=> #{{:key :expected-literal, :at 3, :detail {:literal ")"}}}

(core/parse example) "(42)baz")
=> #{{:key :expected-literal, :at 4, :detail {:literal "foo"}}
     {:key :expected-literal, :at 0, :detail {:literal "bar"}}}

The hard-cut has been replaced with a soft-cut, of which the alias would be >. As shown, this still shows a localized error for the missing ")", yet it also allows backtracking to try the "bar" choice.

Since backtracking before the soft cut is still allowed outside of the chain's scope, the cache is not affected. However, soft and hard cuts can be combined in a grammar. We could for instance extend the grammar a bit more:

(repeat+ (chain example hard-cut))

This effectively says that after each finished example, we won't backtrack, that part is done. Many of such consecutive examples can be parsed, without memory requirements growing. The parse tree does grow of course, though there is an experimental stream+ combinator.

The significance of cuts in PEGs must not be underestimated. Try to use them in your grammar on somewhat larger inputs. The computing overhead is small, and is countered by faster cache lookups.

String-based grammar

A parser or grammar can be defined in a string. While direct combinators have the most flexibility, a string-based definition is far denser. The discussed combinators translate to this string-based grammar in the following way:

literal   <- 'foo'
chain     <- 'foo' 'bar'
choice    <- 'bar' / 'baz'

repeat*   <- 'foo'*
repeat+   <- 'foo'+
maybe     <- 'foo'?

negate    <- !'foo'
lookahead <- &'foo'

regex     <- #'ba(r|z)'
chars     <- [a-zA-Z]*

eof       <- $
ref       <- literal

group     <- ('foo' 'bar' / 'alice')
named     <- (:bax regex)

soft-cut  <- >
hard-cut  <- >>

The function create-parser in the crustimoney.string-grammar is used to create a parser out of such a string. Note that above "example" has rules and thus describes a recursive grammar. Therefore a map is returned by create-parser. However, it is perfectly valid to define a single parser, such as:

'alice and ' !'eve' [a-z]+

The syntax is flexible regarding whitespace. Multiple lines can be on the same line, and a , is also seen as whitespace.

Multiple grammars can be merged, which can come in handy for parsers that are easier expressed in a different way:

(merge
 (create-parser "root <- 'Hello ' email")
 {:email (regex #"...")})

And lastly, the names of the rules can have an = sign appended, for the auto-named feature discussed earlier.

To give an impression on how a string-based grammar looks, below is the same example parser that was used to explain soft-cuts.

( '(' > [0-9]+ ')' )? 'foo' / 'bar'

As you can see, it is far denser, and likely more readable. For more examples, see the examples directory in the library's source.

Data-based grammar

Next to the string-based definition, there is also a data-driven variant available. The grammar below shows how such a definition is formed. It is very similar to the string-based grammar.

'{literal    "foo"
  character  \f
  regex      #"ba(r|z)"
  regex-tag  #crusti/regex "ba(r|z)" ; EDN support

  chain      ("foo" "bar")
  choice     ("bar" / "baz")

  repeat*    ("foo"*)
  repeat+    ("foo"+)
  maybe      ("foo"?)

  negate     (!"foo")
  lookahead  (&"foo")

  eof        $
  ref        literal

  group      ("foo" "bar" / "alice")
  named      (:bax regex)

  soft-cut   >
  hard-cut   >>

  combinator-call   [:with-error {:key :fail} #crusti/parser ("fooba" #"r|z")]
  custom-combinator [:my.app/my-combinator ...]}

The function create-parser in the crustimoney.data-grammar is used to create a parser out of such a definition.

The data-based definition shares many properties with the string-based one. It works the same way in supporting both recursive and non-recursive parsers, it also has auto-naming (the = postfix), and can be used as part of a bigger grammar.

It does have an extra feature: direct combinator calls, using vectors. The first keyword in the vector determines the combinator. If it is without a namespace, crustimoney.combinators is assumed (so not crustimoney.combinator-grammar!). The other arguments are left as-is, except those tagged with #crusti/parser. With that tag, the data is processed again as a parser definition.

Another possible benefit of the data-based grammar over the string-based one, is that is supports nested grammars.

EDN support

Since the grammar definition is data, it is perfectly feasible to use the EDN format. The #crusti/... tags are not automatically supported by Clojure's EDN reader. The following code makes it work:

(clojure.edn/read-string {:readers *data-readers*} ...)

Note that regular expressions are not supported in plain EDN. For this you can use the #crusti/regex tag (see above example), although it could also be written as [:regex {:pattern ".."}].

Vector-based grammar

The former section on data-based grammars describes that a vector is a valid data type, and that these translate to combinator calls. That means that it is possible to write the entire grammar using vectors. Thing is, this is actually what the combinator-based, string-based and data-based parser generators do. They all use such vector model as their output format. This allows the easy combining of multiple grammars and they can be debugged easily.

Another benefit is that the data-grammar can easily be extended. The function data-grammar/vector-tree is from the DataGrammar protocol. This makes it possible to add support for other data types, using Clojure's extend-type. The implementation simply returns a vector, possibly pointing to your own combinator (see further down below).

Built-in parsers

The library provides a couple of predefined parsers in the built-ins namespace, for parsing things like spaces, numbers, words and strings. It also contains a map called all, containing all of the built-in parsers. This map can be used as a basis for your own grammar, by merging them:

(merge built-ins/all (create-parser "
  root <- (space? (:name word) blank (:id natural) space?)* $
"))

Built-in transformer

The results namespace contains mostly basic functions for dealing with the parse results. These include functions as success->text to get the matched text of a node, and success->children to get its children. While not necessary (as the results tree is made of plain vectors), it does increase readability.

Writing your own parse tree processor is easy, as again, it's just data. That said, the results namespace has a transform function you can use. This performs a postwalk, transforming the nodes based on their name, applying a function that receives the node and the full text. Two accompanying helper macros are available, called coerce and collect. Here is an example:

(-> (parse ... text)
    (transform text
      {:number    (coerce parse-long)
       :operand   (coerce {"+" + "-" - "*" * "/" /})
       :operation (collect [[v1 op v2]] (op v1 v2))
       nil        (collect first)}))

If the parse result is not a success, the transform returns the result as is.

The coerce macro creates a transformer, by applying a function to the node's matched text. Instead of a function, coerce can also take a binding vector and a body. So the :number transformation above could be written as (coerce [s] (parse-long s)). It could also be written without the macro as (fn [node text] (parse-long (success->text node text))).

The collect macro also creates a transformation function, by applying a function to the node's children, as seen with the nil (root node) transformer above. Instead of a function, collect can also take a binding vector and a body, as seen with the :operation transformer.

If a transformer is missing, an exception is thrown. For generic transformations (not rule-based), there's a postwalk function.

Experimental combinators

Lastly, there are a couple of experimental combinators. Being experimental, they may get promoted, or changed, or dismissed.

• range, like a repeat, requiring a minimum of matches and stops after a maximum of matches
• stream* and stream+, like repeat*/repeat+, but does not keep its children
• with-callback, fires (success) result of a parser to a callback function

These can be found in the experimental.combinators namespace, including more documentation on them. Note that these experimental combinators compile directly to a parser function, not to the vector model.

Writing your own combinator

A parser combinator returns a function that takes a text input and a position. It returns either a success or (a set of) errors. It does this using the results namespace, which has functions like ->success and ->error. Some combinators take other parser functions as their argument, making them composeable.

However, the parsers returned by the combinators do not call other parsers directly. This could lead to stack overflows. So next to a ->success or ->error result, it can also return a ->push result. This pushes another parser onto a virtual stack, together with an index and possibly some state.

For this reason, a parser function has the following signature:

(fn
  ([text index]
    ...)
  ([text index result state]
   ...))

The 2-arity variant is called when the parser was pushed onto the stack. It receives the entire text and the index it should begin parsing.

If it returns a "push" result, the 4-arity variant is called when that parser is done. It receives the text and the original index, but also the result of the pushed parser and any state that was pushed with it. Now it can decide whether to return a success, a set of errors, or again a push.

Before you write your own combinator, do realise that the provided combinators are complete in the sense that they can parse any structured text.

That's it. As always, have fun! 🚀

License

Copyright © 2022-2024 Arnout Roemers

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at http://www.eclipse.org/legal/epl-2.0.

This Source Code may also be made available under the following Secondary Licenses when the conditions for such availability set forth in the Eclipse Public License, v. 2.0 are satisfied: GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version, with the GNU Classpath Exception which is available at https://www.gnu.org/software/classpath/license.html.