Add basic run command

- Introduced new run command that lets users start an instance of OPA.
- Added basic REPL as first mode that can be run, server mode coming soon.
- Extended Storage to support a JSON Patch like interface.

.gitignore

@@ -1,3 +1,4 @@
+.DS_Store
 .vscode
 opa
 opalog/parser.go

Makefile

@@ -5,7 +5,8 @@
 PACKAGES := \
 	github.com/open-policy-agent/opa/cmd/.../ \
 	github.com/open-policy-agent/opa/eval/.../ \
-	github.com/open-policy-agent/opa/opalog/.../
+	github.com/open-policy-agent/opa/opalog/.../ \
+	github.com/open-policy-agent/opa/runtime/.../
 
 BUILD_COMMIT := $(shell ./build/get-build-commit.sh)
 BUILD_TIMESTAMP := $(shell ./build/get-build-timestamp.sh)

cmd/run.go

@@ -0,0 +1,65 @@
+// Copyright 2016 The OPA Authors.  All rights reserved.
+// Use of this source code is governed by an Apache2
+// license that can be found in the LICENSE file.
+
+package cmd
+
+import (
+	"os"
+	"path"
+
+	"github.com/open-policy-agent/opa/runtime"
+	"github.com/spf13/cobra"
+)
+
+// default filename for the interactive shell's history
+var defaultHistoryFile = ".opa_history"
+
+func init() {
+
+	params := &runtime.Params{}
+
+	runCommand := &cobra.Command{
+		Use:   "run",
+		Short: "Start OPA in interative or server mode",
+		Long: `Start an instance of the Open Policy Agent (OPA).
+
+The 'run' command starts an instance of the OPA runtime. The OPA
+runtime can be started as an interactive shell or a server.
+
+When the runtime is started as a shell, users can define rules and
+evaluate expressions interactively. When the runtime is started as
+a server, users can access OPA's APIs via HTTP.
+
+The runtime can be initialized with one or more JSON files that
+represent base documents.
+`,
+		Run: func(cmd *cobra.Command, args []string) {
+			params.BaseDocPaths = args
+			rt := &runtime.Runtime{}
+			rt.Start(params)
+		},
+	}
+
+	runCommand.Flags().BoolVarP(&params.Server, "server", "s", false, "start the runtime in server mode")
+	runCommand.Flags().StringVarP(&params.HistoryPath, "history", "H", historyPath(), "set path of history file")
+
+	usageTemplate := `Usage:
+  {{.UseLine}} [flags] [files]
+  
+Flags:
+{{.LocalFlags.FlagUsages | trimRightSpace}}
+`
+
+	runCommand.SetUsageTemplate(usageTemplate)
+
+	RootCommand.AddCommand(runCommand)
+}
+
+func historyPath() string {
+	home := os.Getenv("HOME")
+	if len(home) == 0 {
+		return defaultHistoryFile
+	}
+	return path.Join(home, defaultHistoryFile)
+}