Implement parser_chain macro

1 files changed, 44 insertions, 0 deletions

diff --git a/src/parcom.cr b/src/parcom.cr
index cdbebdb..7f0e475 100644
--- a/src/parcom.cr
+++ b/src/parcom.cr
@@ -8,6 +8,50 @@ module Parcom
   class ParserFail < Exception
   end
 
+  # Provides a more convenient syntax for combining parsers via `Parser#and_then`.
+  # The first argument is a string literal used for the name of the parser.
+  # The second and third arguments are types used for the parser's type.
+  # These are followed by any number of 2-tuples containing a variable name and
+  # an expression resolving to a `Parser(t.Class, _)`, whose success value will
+  # be stored in the aformentioned variable. The `finally` named argument is an
+  # expression that resolves to a `Parser(t.class, u.class)`.
+  #
+  # Example:
+  # ```
+  # any_word = Parser(Char, Char).satisfy(&.letter?).some.map(&.join)
+  # ws = Parser(Char, Array(Char)).satisfy(&.whitespace?).many
+  # two_of_same_word = parser_chain "two words", Char, String,
+  #   {word, any_word},
+  #   {_,    ws},
+  #   finally: Parser.token_sequence(word.chars).map(&.join)
+  #
+  # tokens = Tokens.from_string("foo   foo")
+  # result = two_of_same_word.parse(tokens)
+  # result.value # => "foo"
+  #
+  # # The above definition of `two_of_same word`
+  # # is an alternative way of doing this:
+  # two_of_same_word = any_word.and_then do |word|
+  #   ws.and_then do |_|
+  #     Parser.token_sequence(word.chars).map(&.join)
+  #   end
+  # end.named("two words")
+  # ```
+  #
+  # This macro is based on Haskell's do-notation.
+  macro parser_chain(name, t, u, *steps, finally)
+    Parser({{t}}, {{u}}).new({{name}}) do |tokens|
+      {% for tup, index in steps %}
+        {{tup.last}}.and_then do |{{tup.first}}|
+      {% end %}
+      {{finally}}
+      {% for _, _ in steps %}
+        end
+      {% end %}
+      .parse(tokens)
+    end
+  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