first_of

1 files changed, 21 insertions, 0 deletions

diff --git a/src/parcom/parser.cr b/src/parcom/parser.cr
index 0b09e95..95a2e21 100644
--- a/src/parcom/parser.cr
+++ b/src/parcom/parser.cr
@@ -112,6 +112,27 @@ module Parcom
       Parser(T, T).sequence(ps).named("Token Sequence: #{ts}")
     end
 
+    # TODO: Allow support for Iterable(Parser(T, U))
+    def self.first_of(ps : Array(Parser(T, U))) : Parser(T, U)
+      if ps.empty?
+        raise ArgumentError.new("first_of requires at least one parser")
+      end
+
+      Parser(T, U).new("First of: #{ps.map(&.name)}") do |tokens|
+        result = nil
+        ps.each do |p|
+          break unless result.nil?
+          result = result || p.parse?(tokens)
+        end
+
+        if result.nil?
+          raise ParserFail.new("No successes from any provided parsers.")
+        else
+          result
+        end
+      end
+    end
+
     # Creates a new parser from a `Proc`.
     # The `Proc` should have the properties outlined above.
     def initialize(@name : String, @f : Tokens(T) -> Result(T, U))