Implement FirstOf

2 files changed, 57 insertions, 2 deletions

diff --git a/spec/parcom_spec.cr b/spec/parcom_spec.cr
index fd4b470..77a14dd 100644
--- a/spec/parcom_spec.cr
+++ b/spec/parcom_spec.cr
@@ -614,7 +614,38 @@ end
 pending StopIf do
 end
 
-pending FirstOf do
+describe FirstOf do
+  tokens = TokenStream.from_string("abcd")
+  letter_a = Token.new('a')
+  f = Flunk(Char, Char).new
+
+  describe "#parse" do
+    it "cannot be instantiated with an empty Enumerable" do
+      expect_raises(ArgumentError) { FirstOf.new([] of Parser(Char, Char)) }
+    end
+
+    it "uses the result of the first successful parser" do
+      a1 = [letter_a, f, f, f] of Parser(Char, Char)
+      a2 = [f, letter_a, f, f] of Parser(Char, Char)
+      a3 = [f, f, letter_a, f] of Parser(Char, Char)
+      a4 = [f, f, f, letter_a] of Parser(Char, Char)
+
+      [a1, a2, a3, a4].each do |arr|
+        p = FirstOf.new(arr)
+        r = p.parse(tokens)
+        r.value.should eq('a')
+        r.tokens.should eq(tokens[1..])
+      end
+    end
+
+    it "only fails if no parsers are successful" do
+      x = Token.new('x')
+      y = Token.new('x')
+      z = Token.new('x')
+      p = FirstOf.new([x, y, z] of Parser(Char, Char))
+      expect_raises(ParserException) { p.parse(tokens) }
+    end
+  end
 end
 
 pending SepBy do
diff --git a/src/parcom.cr b/src/parcom.cr
index cb3408a..2a61c2d 100644
--- a/src/parcom.cr
+++ b/src/parcom.cr
@@ -405,7 +405,31 @@ module Parcom
   class StopIf
   end
 
-  class FirstOf
+  class FirstOf(T, V) < Parser(T, V)
+    @p : Parser(T, V)
+
+    def initialize(ps : Iterable(Parser(T, V)))
+      ps_iter = ps.each
+      p = ps_iter.next
+
+      if p.is_a?(Iterator::Stop)
+        raise ArgumentError.new("FirstOf requires atleast one parser, got none")
+      end
+
+      @p = p
+      p = ps_iter.next
+
+      until p.is_a?(Iterator::Stop)
+        @p = @p | p
+        p = ps_iter.next
+      end
+    end
+
+    def parse(tokens : TokenStream(T)) : Result(T, V)
+      @p.parse(tokens)
+    rescue ex : ParserException
+      raise ParserException.new("FirstOf: #{ex.message}")
+    end
   end
 
   class SepBy