Add nomove state transition option, session partitioned interactive example

examples/session/input

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+hey hey hey
+your data is {{.do_save}}

examples/session/input.vis

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+MAP do_save
+HALT
+RELOAD do_save
+INCMP * .

examples/session/main.go

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+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"path"
+
+	testdataloader "github.com/peteole/testdata-loader"
+
+	"git.defalsify.org/vise/cache"
+	"git.defalsify.org/vise/engine"
+	"git.defalsify.org/vise/resource"
+	"git.defalsify.org/vise/state"
+)
+
+var (
+	baseDir = testdataloader.GetBasePath()
+	scriptDir = path.Join(baseDir, "examples", "session")
+	emptyResult = resource.Result{}
+)
+
+func save(sym string, input []byte, ctx context.Context) (resource.Result, error) {
+	sessionId := ctx.Value("SessionId").(string)
+	sessionDir := path.Join(scriptDir, sessionId)
+	err := os.MkdirAll(sessionDir, 0700)
+	if err != nil {
+		return emptyResult, err
+	}
+	fp := path.Join(sessionDir, "data.txt")
+	if len(input) > 0 {
+		log.Printf("write data %s session %s", input, sessionId)
+		err = ioutil.WriteFile(fp, input, 0600)
+		if err != nil {
+			return emptyResult, err
+		}
+	}
+	r, err := ioutil.ReadFile(fp)
+	if err != nil {
+		err = ioutil.WriteFile(fp, []byte("(not set)"), 0600)
+		if err != nil {
+			return emptyResult, err
+		}
+	}
+	return resource.Result{
+		Content: string(r),	
+	}, nil
+}
+
+func main() {
+	var root string
+	var size uint
+	var sessionId string
+	flag.UintVar(&size, "s", 0, "max size of output")
+	flag.StringVar(&root, "root", "root", "entry point symbol")
+	flag.StringVar(&sessionId, "session-id", "default", "session id")
+	flag.Parse()
+	fmt.Fprintf(os.Stderr, "starting session at symbol '%s' using resource dir: %s\n", root, scriptDir)
+
+	st := state.NewState(0)
+	rs := resource.NewFsResource(scriptDir)
+	rs.AddLocalFunc("do_save", save)
+	ca := cache.NewCache()
+	cfg := engine.Config{
+		Root: "root",
+		SessionId: sessionId,
+		OutputSize: uint32(size),
+	}
+	ctx := context.Background()
+	ctx = context.WithValue(ctx, "SessionId", sessionId)
+	en := engine.NewEngine(cfg, &st, rs, ca, ctx)
+	err := en.Init(ctx)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "engine init fail: %v\n", err)
+		os.Exit(1)
+	}
+	err = engine.Loop(&en, os.Stdin, os.Stdout, ctx)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "loop exited with error: %v\n", err)
+		os.Exit(1)
+	}
+}

examples/session/root.vis

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+LOAD do_save 0
+MOVE input