first_of

2 files changed, 51 insertions, 1 deletions

diff --git a/spec/parcom_spec.cr b/spec/parcom_spec.cr
index aae1d1b..af95953 100644
--- a/spec/parcom_spec.cr
+++ b/spec/parcom_spec.cr
@@ -216,6 +216,36 @@ describe Parser do
     end
   end
 
+  describe "self.first_of" do
+    ps = [
+      Parser.token('a'),
+      Parser.token('b'),
+      Parser.token('c'),
+    ]
+    abc = Parser(Char, Char).first_of(ps)
+
+    it "fails to instantiate if array is empty" do
+      expect_raises(ArgumentError) { Parser.first_of([] of Parser(Char, Char)) }
+    end
+
+    it "returns the result of the first parser that succeeds" do
+      {"a", "b", "c"}.each do |s|
+        tokens = Tokens.from_string(s)
+        result = abc.parse(tokens)
+
+        result.value.should eq(s[0])
+        result.tokens.empty?.should be_true
+      end
+    end
+
+    it "fails if none of the parsers succeed" do
+      {"", "d"}.each do |s|
+        tokens = Tokens.from_string(s)
+        expect_raises(ParserFail) { abc.parse(tokens) }
+      end
+    end
+  end
+
   describe "#assert" do
     p = Parser(Char, Char).any_token.assert { |c| c == 'a' }
 
@@ -621,7 +651,6 @@ describe Parser do
     end
   end
 
-  # TODO: first_of
   # TODO: sep_by
   # TODO: phrase
   # TODO: peek
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))