Initial Commit

1 files changed, 43 insertions, 0 deletions

diff --git a/patter.rb b/patter.rb
new file mode 100644
index 0000000..348cbdf
--- /dev/null
+++ b/patter.rb
@@ -0,0 +1,43 @@
+module Patter
+  class Var; end
+
+  class Fun
+    def initialize
+      @patterns = {}
+    end
+
+    def when(*args, &block)
+      raise "Block arity does not match number of free variables" if args.count(Var) != block.arity
+      @patterns[args] = block
+    end
+
+    def call(*args)
+      free_vars, block = match(args)
+      block.call(*free_vars)
+    end
+
+    alias_method :[], :call
+
+    private
+
+    def match(args)
+      @patterns.each do |pattern, block|
+        next if pattern.length != args.length
+        free_variables = []
+        matched = true
+        pattern.zip(args).each do |pair|
+          if pair.first == Var then
+            free_variables << pair.last
+            next
+          end
+          if pair.first != pair.last then
+            matched = false
+            break
+          end
+        end
+        return [free_variables, block] if matched
+      end
+      raise "Inexhaustive patterns"
+    end
+  end
+end