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pars3k (parsec/parse 3000) is a library for Crystal adding support for combinator parsers. The structure is heavily inspired by Parsec from Haskell. It is also inspired by crystal-parsec.


A combinator parser is a parsing system that allows for the creation of small "parsers," an object that takes a String in, and has some small computational logic applied to extract a deeper meaning, like parsing through JSON and converting to a usable Hash.

The idea of a combinator parser is the concept of literally combining parsers. Small parsers can be combined with logic (like OR, AND, etc.) to create larger, more meaningful parsers. Ultimately, you should end up with one big parser.


As mentioned before, this style of parser can be used for creating interpreted programming languages, decoding markup languages, reading from files of different formats, etc. It is generally accepted as a more flexible and reliable parsing style than, say, regular expressions. The following category will contain some use cases using pars3k.


require "pars3k"
include Pars3k

Note: HIGHLY recommend you include Pars3k

All parsers are of type Parser(T), where T is the ultimate return type of the Parser. One example of a primitive Parser is a Parser(Char), which can be created using Parse.char(Char). The method gives a Parser(Char) that, when parsed, looks for a specific character as specified.

Primitive parsers

char_a = Parse.char('a')

puts char_a.parse "abc" #=> a

This example creates a Parser(Char) from Parse.char, and parses the string "abc" on it. The character parser looks at the beginning of the string, and looks for the first character. If the first character matches the character supplied to the parser originally (Parse.char 'a'), then the parse will succeed and the parse result will return the character that was matched.

puts char_a.parse "bca" #=> expected 'a', got 'b'

This example uses the same char_a parser, but parses string "bca" on it. Because it doesn't start with the original requirement 'a', the parse fails and returns a ParseError. A ParseError is a struct that contains a message about the parse failure, which can be retrieved using ParseError#message. As such, the return type of Parser(T)#parse is a union type of (T | ParseError), as it can return either.

str_cat = Parse.string "cat"

puts str_cat.parse "cat" #=> cat
puts str_cat.parse "cats are cool" #=> cat
puts str_cat.parse "dog" #=> expected 'cat', got 'd'

This example creates a new primitive parser, the Parser(String) created by Parse.string(String). It expects an exact copy of the string provided. In this example, the text "cat" is used.

Amalgam parsers

char_a = Parse.char 'a'
char_b = Parse.char 'b'
parse_ab = char_a | char_b

puts parse_ab.parse "abc" #=> a
puts parse_ab.parse "bca" #=> b
puts parse_ab.parse "cab" #=> expected 'b', got 'c'

This example creates three parsers:

  • a Parser(Char) that expects a character of 'a',
  • a Parser(Char) that expects a character of 'b', and
  • a Parser(Char) created using the | operator that will try the left parser first, then the right, and use the successful parser.

As seen the | operator allows you to create amalgam parsers by using OR logic. It first tries the parser on the left, then the right. If both fail, it will throw the ParseError given by the rightmost parser.

This process is tedious for large masses of characters, such as if you wanted to accept all letters of the alphabet. For this sake, there exists a Parser(Char) that can be created with Parse.one_char_of(String), which accepts a String as a list of characters, and allows a character to be parsed that is in that list.

parse_alphabet = Parse.one_char_of "abcdefghijklmnopqrstuvwxyz"

puts parse_alphabet.parse "abc" #=> a
puts parse_alphabet.parse "bca" #=> b
puts parse_alphabet.parse "xyz" #=> x
puts parse_alphabet.parse "yzx" #=> y
puts parse_alphabet.parse "123" #=> expected 'z', got '1'

This example creates a parser that accepts a char from the provided list. As seen, alphabetical characters parse successfully, but numerical characters do not, as they were not in the original string of the alphabet.

Additionally, this parse_alphabet definition is available from Parse.alphabet, which also accepts uppercase. Parse.alphabet_lower only accepts lowercase.

Repetitive parsers

We can create parsers that repeat until they no longer parse using Parse.many_of(Parser(T)). This parser takes in any kind of Parser(T), and outputs a usable parser of type Parser(Array(T)). It will match a string continuously until it no longer can, and group all parsed values into an array.

Extra note: Parse.one_or_more_of(Parser(T)) also exists, acting similarly, but erroring if at least one parse does not parse successfully.

word = Parse.many_of Parse.alphabet

puts word.parse "hello world" #=> ['h', 'e', 'l', 'l', 'o']
puts word.parse "abc" #=> ['a', 'b', 'c']
puts word.parse "123" #=> []

There is a clear issue with the result of the parser: it returns a list of the characters. If we want to convert this into a usable String, we have to transform the parser.

Transforming parsers

Existing parsers can be "transformed" to create new parsers with new logic. Transforming parsers is very useful. To transform a parser, use the Parser(T)#transform(T -> B) method, which accepts a block that receives the resulting value of a parse as a parameter, and outputs a transformed/mapped value.

For example, if you created a parser that accepted numbers:

digit = Parse.one_char_of "0123456789"

Upon parsing it, it would yield characters on success:

puts (digit.parse "1").class #=> Char

we find that the result is a Char, not any form of a Number! To solve this, we can transform the parser:

digit = (Parse.one_char_of "0123456789").transform { |char| char.to_i }

puts digit.parse "1" #=> 1
puts (digit.parse "1").class #=> Int32

Success! Now the parsed value from our parser is the correct type, Int32.

Back to the issue we found in the word parser from the previous section, we can transform the Array(Char) to a String.

word = (Parse.many_of Parse.alphabet).transform &.reduce("") { |value, char| value + char}

puts word.parse "hello world" #=> hello
puts word.parse "abc" #=> abc
puts word.parse "" #=>

This transformation takes chars, an Array(Char), and transforms it by using its reduce method, which iterates through all of the characters in the array and adds them to a blank string.

Additionally, this identical word parser is available as Parse.word (Parser(String)).

Parser operators

The | (OR) operator has already been discussed. There are a couple more operators, like:

  • A >> B creates a new parser that ensures both A and B parse sequentially, but results with the value of B.
  • A << B creates a new parser that ensures both A and B parse sequentially, but results with the value of A.
letter = Parse.alphabet
digit = Parse.one_char_of "0123456789"
parser_take_digit = letter >> digit
parser_take_letter = letter << digit

puts parser_take_digit.parse "a1" #=> 1
puts parser_take_digit.parse "b2" #=> 2

puts parser_take_letter.parse "a1" #=> a
puts parser_take_letter.parse "b2" #=> b

In this example, two parsers are created, letter and digit, that are just amalgam parsers that allow any alphabetical character or any digit character, respectively. Then, two new parsers are created using the >> and << operators. The first parses both sequentially but results with the result of digit, and the second does the same but results with the value of letter. Upon parsing these, the two parsers must work sequentially, but only returns with the parser's result the operator is pointing toward.

Parsing lists

Parse has a special parser that can parse a list of parsable items by parser A, delimited by parser B. Using this, we can create a parser that parses through a list of words (using Parser.word), delimited by an amalgam parser that uses commas. It is called from Parse.delimited_list(Parser(A), Parser(B)) : Parser(Array(A)).

word = Parser.word
optional_whitespace = Parser.many_of Parser.char ' '
comma = (Parser.char ',') << optional_whitespace

list_parser = Parse.delimited_list word, comma

puts list_parser.parse "hello, world" #=> ["hello", "world"]
puts list_parser.parse "how,are,    you" #=> ["how", "are", "you"]
puts list_parser.parse "123, 456" #=> []
puts list_parser.parse "hello world, how are you" #=> ["hello"]

Complex sequential parsers

In the event you need to create complex sequential parsers, you can use Parser(T)#sequence. The sequence method is a method that takes in a block that receives the output of Parser(T) as a value, and must return a new Parser of any type, or Parser(B). We can recreate the parser_take_digit and parser_take_letter parsers using this functionality:

letter = Parse.alphabet
digit = Parse.one_char_of "0123456789"

parser_take_digit = letter.sequence do |char_result|
  digit.sequence do |digit_result|
    Parse.constant digit_result

The original two parsers are sequenced in each other, and ultimately, a Parse.constant parser is returned. A Parse.constant parser is a parser that takes in any value of type T. When parsed, it ALWAYS returns the value of type T. In this case, we create it with the Char result from digit.

parser_letter_digit = letter.sequence do |char_result|
  digit.sequence do |digit_result|
    Parse.constant({char_result, digit_result}) # a constant parser with a `Tuple(Char, Char)`

This parser will parse strings like a1, b2, c3, etc., but return both of the retrieved values as a Tuple.

result = parser_letter_digit.parse "a1"

puts result[0] #=> a
puts result[1] #=> 1

This form of parser sequencing can quickly become tedious. As a result, the library has a special macro inspired by Haskell's do statement. It allows you to chain parsers like above, but in a much more linear and organized manner. Here is the most recent sequential parser parser_letter_digit using do_parse:

parser_letter_digit = do_parse({
  char_result <= letter,
  digit_result <= digit,
  Parse.constant({char_result, digit_result})

The body of the do_parse macro is a list of actions separated by commas. The last element of this list MUST be an expression that is ultimately returned through the new parser.

For each of the other elements in the list, they must be either parser results or local variables.

  • Parser results look like result_variable_name <= parser,. In this case, the result from parser is stored as result_variable_name.
  • Local variables are simply variable_name = value,. In this case, variable_name is set to value.

Utilizing our knowledge of pars3k, we can create parsers like this:

word = Parse.word

whitespace = Parse.many_of Parse.char ' '
equals = whitespace >> (Parse.char '=') << whitespace

key_value_pair = do_parse({
  key <= word,
  eq <= equals,
  value <= word,
  Parse.constant({key, value})

comma = (Parse.char ',') << whitespace

key_value_list = Parse.delimited_list key_value_pair, comma

puts key_value_list.parse "hello = world" #=> [{"hello", "world"}]
puts key_value_list.parse "how = are, you= sir" #=> [{"how", "are"}, {"you", "sir"}]
puts key_value_list.parse "all=     sorts,of   =supported, white = spaces" #=> [{"all", "sorts"}, {"of", "supported"}, {"white", "spaces"}]

Custom parsers

Custom parsers can be created using custom logic. This is sometimes necessary if existing primitive parsers cannot be combined effectively enough to accomplish a task.

def char_parser(char)
  Parser(Char).new do |context|
    if context.position >= context.parsing.size
      ParseResult(Char).error "expected '#{char}', got end of string", context
    elsif context.parsing[context.position] == char
      ParseResult(Char).new char,
      ParseResult(Char).error "expected '#{char}', got '#{context.parsing[context.position]}", context
  • context is a ParseContext
  • context.parsing is the string being parsed
  • context.position is the position in the string being analyzed
  • ParseResult(T) is a parse result of type T, same as the parser expected
  • ParseResult(T).new result, new_context_after_shift should be used to yield parsed values
  • ParseResult(T).error message, original_context should be used to throw parse errors

This defines char_parser(Char), which creates a parser that expects a character as specified. This implementation is the same as the internal implementation Parse.char(Char). See the source code for more applications of Parsers derived from blocks.


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Crystal parser combinator library




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