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require "./parcom/*"
module Parcom
VERSION = "0.1.0"
# A ParserFail exception should be raised by `Parser#parse` when
# a parse attempt is unsuccessful.
class ParserFail < Exception
end
# `Tokens` is an `Array` wrapper struct to store the input
# stream of one or more `Parser` objects.
# A `Tokens` can be created from any `Iterable`, along with
# `String` objects using a special constructor.
struct Tokens(T)
getter tokens
# Constructs a `Tokens(Char)` from a `String`.
def self.from_string(s : String) : Tokens(Char)
Tokens.new(s.chars)
end
# Constructs a `Tokens` from an `Iterable`.
def initialize(ts : Iterable(T))
if ts.responds_to?(:to_a)
@tokens = ts.to_a
else
@tokens = [] of T
ts.each { |t| @tokens << t }
end
end
# Exposes `Array#[](Int)`.
def [](index : Int) : T
@tokens[index]
end
# Exposes `Array#[](Int, Int)`, but wraps the returned array in a new `Tokens`.
def [](start : Int, count : Int) : Tokens(T)
Tokens.new(@tokens[start, count])
end
# Exposes `Array#[](Range)`, but wraps the returned array in a new `Tokens`.
def [](range : Range) : Tokens(T)
Tokens.new(@tokens[range])
end
# Like `#[]`, but returns `nil` instead of raising an `IndexError`.
def []?(*args)
self.[](*args)
rescue IndexError
nil
end
# Exposes `Array#empty?`.
def empty? : Bool
@tokens.empty?
end
end
# A `Result` stores a `Tokens` object and a parsed value,
# and is effectively used to store the state of a parser chain.
# This is used instead of a `Tuple` or `NamedTuple` because:
# 1. This is more idiomatic than a `Tuple`.
# 2. Crystal does not support generic named tuples.
struct Result(T, U)
getter tokens, value
def initialize(@tokens : Tokens(T), @value : U)
end
def map(f : U -> V) : Result(T, V) forall V
Result.new(@tokens, f.call(@value))
end
def map(&block : U -> V) : Result(T, V) forall V
map(block)
end
end
# A parser defines a process of extracting some sort of data
# from a stream of tokens.
# `Parser` objects wrap a function/block that accepts a `Tokens` object,
# and either returns a `Result`, or raises a `ParserFail`.
# This struct also defines a handful of methods for modifying a parser's
# behavior.
#
# The function that is wrapped by a parser should:
# 1. Have the type `Tokens(T) -> Result(T, U)`
# 1. Indicate failure by raising a `ParserFail`
# 1. Not introduce side-effects
#
# Instead of inheriting from `Parser`, custom parsers should be
# defined using a method that generates the parser directly:
# ```
# struct Foo(T, U) < Parser(T, Array(U))
# def parse(tokens)
# # This will cause headaches
# end
# end
#
# # Do this instead:
# def foo(t : T.class, u : U.class, p : Parser(T, U)) : Parser(T, U) forall T, U
# Parser(T, U).new("Foo") { |tokens| "your code here" }
# end
# ```
struct Parser(T, U)
getter name
def initialize(@name : String, @f : Tokens(T) -> Result(T, U))
end
def initialize(@name : String, &block : Tokens(T) -> Result(T, U))
@f = block
end
# Changes the `name` property and returns `self`.
# This should be used to specify a custom name:
# ```
# a = Basic.token(Char, 'a').named("letter a")
# ```
def named(name : String) : self
@name = name
self
end
def parse(tokens : Tokens(T)) : Result(T, U)
@f.call(tokens)
end
def parse?(tokens : Tokens(T)) : Result(T, U)?
parse(tokens)
rescue
nil
end
def assert(f : U -> Bool) : Parser(T, U)
p = self
Parser.new("#{p.name} (assertion)") do |tokens|
result = p.parse(tokens)
unless f.call(r.value)
raise ParserFail.new("Assertion failed for value #{r.value}")
end
result
end
end
def assert(&block : U -> Bool) : Parser(T, U)
assert(block)
end
def map(f : U -> T) : Parser(T, V) forall V
p = self
Parser(T, V).new("#{p.name} (mapped)") do |tokens|
p.parse(tokens).map(f)
end
end
def map(&block : U -> T) : Parser(T, V) forall V
map(block)
end
end
end
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