Add 'make' and 'pure' arguments to parser_chain

1 files changed, 29 insertions, 15 deletions

diff --git a/src/parcom.cr b/src/parcom.cr
index 0963184..60b8df0 100644
--- a/src/parcom.cr
+++ b/src/parcom.cr
@@ -12,9 +12,13 @@ module Parcom
   # The first and second arguments are types used for the parser's type.
   # The thirs argument is a string literal used for the name of the parser.
   # This is 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)`.
+  # an expression resolving to a `Parser(t.class, _)`, whose success value will
+  # be stored in the aformentioned variable. The `make` named argument is an
+  # expression that resolves to a `Parser(t.class, u.class)`. This parser is
+  # the final step of the parser chain. The `pure` named argument is an
+  # expression that resolves to a `u.class`. This expression is passed to a
+  # call to `Parser(t.class, u.class).pure`, which will be the final step
+  # of the parser chain.
   #
   # Example:
   # ```
@@ -23,7 +27,7 @@ module Parcom
   # two_of_same_word = parser_chain Char, String, "two words",
   #   {word, any_word},
   #   {_,    ws},
-  #   finally: Parser.token_sequence(word.chars).map(&.join)
+  #   make: Parser.token_sequence(word.chars).map(&.join)
   #
   # tokens = Tokens.from_string("foo   foo")
   # result = two_of_same_word.parse(tokens)
@@ -39,17 +43,27 @@ module Parcom
   # ```
   #
   # This macro is based on Haskell's do-notation.
-  macro parser_chain(t, u, name, *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
+  macro parser_chain(t, u, name, *steps, make = nil, pure = nil )
+    {% if make.nil? && pure.nil? %}
+      raise ArgumentError.new("Expected exactly one of 'make' and 'pure', but got neither")
+    {% elsif !make.nil? && !pure.nil? %}
+      raise ArgumentError.new("Expected exactly one of 'make' and 'pure', but got both")
+    {% else %}
+      Parser({{t}}, {{u}}).new({{name}}) do |tokens|
+        {% for tup, index in steps %}
+          {{tup.last}}.and_then do |{{tup.first}}|
+        {% end %}
+        {% if !make.nil? %}
+          {{make}}
+        {% else %}
+          Parser({{t}}, {{u}}).pure({{pure}})
+        {% end %}
+        {% for _, _ in steps %}
+          end
+        {% end %}
+        .parse(tokens)
+      end
+    {% end %}
   end
 end