Add param permitted

lib/tty/option.rb

@@ -43,6 +43,9 @@ module Option
     # Raised when conversion type isn't registered
     UnsupportedConversion = Class.new(Error)
 
+    # Raised when argument value isn't permitted
+    UnpermittedArgument = Class.new(Error)
+
     # Enhance object with command line option parsing
     #
     # @api public

lib/tty/option/param_permitted.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+module TTY
+  module Option
+    module ParamPermitted
+      # Convert parameter value to another type
+      #
+      # @example
+      #   param = Parameter::Argument.create(:foo, permit: %w[11 12 13])
+      #   ParamPermitted[param, "12"] # => 12
+      #
+      # @api public
+      def call(param, value)
+        return value unless param.permit?
+
+        if param.permit.include?(value)
+          value
+        else
+          raise UnpermittedArgument,
+                format("unpermitted argument %s for %s parameter",
+                       value, param.name.inspect)
+        end
+      end
+      module_function :call
+
+      alias :[] :call
+      module_function :[]
+    end # ParamPermitted
+  end # Option
+end # TTY

lib/tty/option/pipeline.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require_relative "param_conversion"
+require_relative "param_permitted"
 require_relative "param_validation"
 
 module TTY

spec/unit/param_permitted_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+RSpec.describe TTY::Option::ParamPermitted do
+  it "skips check when no permit setting" do
+    param = TTY::Option::Parameter::Option.create(:foo)
+
+    expect(described_class[param, "a"]).to eq("a")
+  end
+
+  it "permits an option argument" do
+    param = TTY::Option::Parameter::Option.create(:foo, permit: %w[a b c])
+
+    expect(described_class[param, "a"]).to eq("a")
+  end
+
+  it "doesn't permit an option arguemnt" do
+    param = TTY::Option::Parameter::Option.create(:foo, permit: %w[a b c])
+
+    expect {
+      described_class[param, "d"]
+    }.to raise_error(TTY::Option::UnpermittedArgument,
+                    "unpermitted argument d for :foo parameter")
+  end
+end