Am I doing this right? #39

dlech · 2017-02-15T23:50:13Z

I'm inexperienced in go, so I thought I would share my first ev3go program and see what you think.

package main

import (
	"log"

	"strconv"

	"image/color"

	"os/exec"

	"github.com/ev3go/ev3"
	"github.com/ev3go/ev3dev"
)

const (
	top    = 30
	bottom = 100
	left   = 70
	right  = 110
)

func main() {
	// configure the LEGO EV3 IR sensor on input port 1 in IR-SEEK mode
	irSensor, err := ev3dev.SensorFor("in1", "lego-ev3-ir")
	if err != nil {
		log.Fatal(err)
	}
	irSensor.SetMode("IR-SEEK")

	// configure LEGO EV3 Large motor in output port A
	motor, err := ev3dev.TachoMotorFor("outA", "lego-ev3-l-motor")
	if err != nil {
		log.Fatal(err)
	}

	// init poller so we can monitor buttons
	buttonPoller := ev3dev.ButtonPoller{}

	// init the LCD so we can draw stuff
	ev3.LCD.Init(true)
	defer ev3.LCD.Close()

	motor.Command("run-direct")
	beaconPresent := false

	for {
		// read the channel 1 heading value from the IR sensor
		v0, _ := irSensor.Value(0)
		heading, _ := strconv.Atoi(v0)

		// motor rotation is in the same direction as heading, so this will
		// cause the motor to rotate towards the beacon
		motor.SetDutyCycleSetpoint(heading)

		v1, _ := irSensor.Value(1)
		distance, _ := strconv.Atoi(v1)

		// sound feedback when beacon is in or out of range
		if beaconPresent && distance == -128 {
			beaconPresent = false
			exec.Command("beep", "-f", "400", "-n", "-f", "200").Start()
		} else if !beaconPresent && distance != -128 {
			beaconPresent = true
			exec.Command("beep", "-f", "200", "-n", "-f", "400").Start()
		}

		// Draw a bar graph to indicate how far away the beacon is. The closer
		// the beacon, the taller the graph.
		for x := left; x < right; x++ {
			for y := top; y < bottom; y++ {
				if y > top+distance {
					ev3.LCD.Set(x, y, color.Black)
				} else {
					ev3.LCD.Set(x, y, color.White)
				}
			}
		}

		// check for button presses
		b, err := buttonPoller.Poll()
		if err != nil {
			log.Fatal(err)
		}
		if (b & ev3dev.Back) == ev3dev.Back {
			// exit the program
			break
		}
		if (b & ev3dev.Middle) == ev3dev.Middle {
			speak := exec.Command("espeak", "--stdout", "-a", "200", "-s", "100",
				"Don't press that button")
			play := exec.Command("aplay", "-q")
			play.Stdin, _ = speak.StdoutPipe()
			speak.Start()
			play.Start()
		}
	}
}

kortschak · 2017-02-15T23:59:05Z

A quick read; it looks OK to me. There are some C-isms there 😁. If you'd like I can massage it to how I would express it when I'm home from work and then you can take what you like and leave what you don't.

dlech · 2017-02-16T00:04:48Z

That would be great. I really like go so far, the little bit that I have used it.

kortschak · 2017-02-16T00:11:42Z

Yeah, I noticed ev3dev/lmsasm. Nice.

kortschak · 2017-02-16T10:32:44Z

I've left commentary, though added some where I thought it would be useful for you.

package main

import (
	"image/color"
	"log"
	"os/exec"
	"strconv"

	"github.com/ev3go/ev3"
	"github.com/ev3go/ev3dev"
)

func main() {
	log.Println("Hello World!")

	// configure the LEGO EV3 IR sensor on input port 1 in IR-SEEK mode
	irSensor, err := ev3dev.SensorFor("in1", "lego-ev3-ir")
	if err != nil {
		log.Fatal(err)
	}
	// We could capture the error here and finish
	// early but it doesn't really matter. If more
	// work were being done, I would fail here.
	irSensor.SetMode("IR-SEEK")

	// configure LEGO EV3 Large motor in output port A
	motor, err := ev3dev.TachoMotorFor("outA", "lego-ev3-l-motor")
	if err != nil {
		log.Fatal(err)
	}
	// Same here... it will be caught in the loop.
	motor.Command("run-direct")

	// init the LCD so we can draw stuff
	ev3.LCD.Init(true)
	// This will only happen in the happy path since
	// defered function will not be called if we
	// log.Fatal, but the file will be closed by the
	// kernel in that case. Here the defer is really
	// politeness more than anything else.
	defer ev3.LCD.Close()

	seen := false
	for !done() {
		// read the channel 1 heading value from the IR sensor
		heading, err := intValueFrom(irSensor, 0)
		if err != nil {
			log.Fatal(err)
		}

		// motor rotation is in the same direction as heading, so this will
		// cause the motor to rotate towards the beacon
		err = motor.SetDutyCycleSetpoint(heading).Err()
		if err != nil {
			log.Fatal(err)
		}

		distance, err := intValueFrom(irSensor, 1)
		if err != nil {
			log.Fatal(err)
		}
		seen, err = checkBeacon(seen, distance)
		if err != nil {
			log.Fatal(err)
		}

		// Draw a bar graph to indicate how far away the beacon is. The closer
		// the beacon, the taller the graph.
		plot(distance)
	}
}

func intValueFrom(s *ev3dev.Sensor, i int) (int, error) {
	v, err := s.Value(i)
	if err != nil {
		return -1, err
	}
	return strconv.Atoi(v)
}

var (
	fallingTones = []string{"-f", "400", "-n", "-f", "200"}
	risingTones  = []string{"-f", "200", "-n", "-f", "400"}
)

func checkBeacon(wasPresent bool, distance int) (seen bool, err error) {
	// sound feedback when beacon is in or out of range
	if wasPresent && distance == -128 {
		err = exec.Command("beep", fallingTones...).Start()
		return false, err
	} else if !wasPresent && distance != -128 {
		err = exec.Command("beep", risingTones...).Start()
		return true, err
	}
	return wasPresent, nil
}

func plot(distance int) {
	const (
		top    = 30
		bottom = 100
		left   = 70
		right  = 110
	)

	for x := left; x < right; x++ {
		for y := top; y < bottom; y++ {
			if y > top+distance {
				ev3.LCD.Set(x, y, color.Black)
			} else {
				ev3.LCD.Set(x, y, color.White)
			}
		}
	}
}

func say(text string) error {
	speak := exec.Command("espeak", "--stdout", "-a", "200", "-s", "100", text)
	play := exec.Command("aplay", "-q")
	var err error
	play.Stdin, err = speak.StdoutPipe()
	if err != nil {
		return err
	}
	err = speak.Start()
	if err != nil {
		return err
	}
	return play.Start()
}

var buttons ev3dev.ButtonPoller

func done() bool {
	b, err := buttons.Poll()
	if err != nil {
		log.Fatal(err)
	}
	if b&ev3dev.Back != 0 {
		return true
	}
	if b&ev3dev.Middle != 0 {
		err = say("Don't press that button")
		if err != nil {
			log.Fatal(err)
		}
	}
	return false
}

The use of fatal over panic and recovery is something I am not yet convinced of either way.

One thing that is odd is that when I run this, I get the following failure after variable amounts of time:

2017/02/16 10:15:21 Hello World!
2017/02/16 10:15:27 ev3dev: failed to read sensor0 value0 attribute /sys/class/lego-sensor/sensor0/value0: read /sys/class/lego-sensor/sensor0/value0: no such device or address at ev3dev.go:469 github.com/ev3go/ev3dev.attributeOf

It works for a while, then the sensor attribute seems to disappear, though it comes back. Do you have any ideas what might be causing that?

kortschak · 2017-02-16T11:57:29Z

Ahh, it looks to me like it was a problem with the old kernel I was using - ~~though now I have lost wireless (rtl8188eu-modules-4.4.47-19-ev3dev-ev3 is installed correctly).~~ Running through the connmanctl process brought that back.

dlech · 2017-02-16T18:42:53Z

Thanks for the feedback. I think panic actually makes very good sense when working with motors. For example, I am now doing this:

err = motor.Command("run-direct").Err()
if err != nil {
	log.Panic(err)
}
defer motor.Command("reset")

So, if something bad happens (like a sensor comes unplugged, the motor will stop when the program exits because of the panic.

kortschak · 2017-02-16T21:28:06Z

You can do it more easily since panic unwinds the stack. Conventionally you use a must function.

func must(err error) {
    if err != nil {
        panic(err)
    }
}

This gives you something like this (the balance of when to add a new must is up to taste - this might want a mustInt if there were more cases where we returned a (int, err).

package main

import (
	"image/color"
	"log"
	"os/exec"
	"strconv"

	"github.com/ev3go/ev3"
	"github.com/ev3go/ev3dev"
	"github.com/ev3go/ev3dev/motorutil"
)

func main() {
	log.Println("Hello World!")

	// configure the LEGO EV3 IR sensor on input port 1 in IR-SEEK mode
	irSensor, err := ev3dev.SensorFor("in1", "lego-ev3-ir")
	if err != nil {
		log.Fatal(err)
	}
	// We could capture the error here and finish
	// early but it doesn't really matter. If more
	// work were being done, I would fail here.
	irSensor.SetMode("IR-SEEK")

	// Set up defer recover to handle motor resets on failure.
	defer func() {
		r := recover()
		if r != nil {
			err := motorutil.ResetAll()
			if err != nil {
				log.Printf("failed to reset all motors: %v", err)
			}
			log.Fatal(r)
		}
	}()

	// configure LEGO EV3 Large motor in output port A
	motor, err := ev3dev.TachoMotorFor("outA", "lego-ev3-l-motor")
	if err != nil {
		log.Fatal(err)
	}
	must(motor.Command("run-direct").Err())

	// init the LCD so we can draw stuff
	ev3.LCD.Init(true)
	defer ev3.LCD.Close()

	seen := false
	for !done() {
		// read the channel 1 heading value from the IR sensor
		heading := intValueFrom(irSensor, 0)

		// motor rotation is in the same direction as heading, so this will
		// cause the motor to rotate towards the beacon
		must(motor.SetDutyCycleSetpoint(heading).Err())

		distance := intValueFrom(irSensor, 1)
		seen = checkBeacon(seen, distance)

		// Draw a bar graph to indicate how far away the beacon is. The closer
		// the beacon, the taller the graph.
		plot(distance)
	}
}

func must(err error) {
	if err != nil {
		panic(err)
	}
}

func intValueFrom(s *ev3dev.Sensor, i int) int {
	v, err := s.Value(i)
	if err != nil {
		panic(err)
	}
	i, err = strconv.Atoi(v)
	if err != nil {
		panic(err)
	}
	return i
}

var (
	fallingTones = []string{"-f", "400", "-n", "-f", "200"}
	risingTones  = []string{"-f", "200", "-n", "-f", "400"}
)

func checkBeacon(wasPresent bool, distance int) (seen bool) {
	// sound feedback when beacon is in or out of range
	if wasPresent && distance == -128 {
		must(exec.Command("beep", fallingTones...).Start())
		return false
	} else if !wasPresent && distance != -128 {
		must(exec.Command("beep", risingTones...).Start())
		return true
	}
	return wasPresent
}

func plot(distance int) {
	const (
		top    = 30
		bottom = 100
		left   = 70
		right  = 110
	)

	for x := left; x < right; x++ {
		for y := top; y < bottom; y++ {
			if y > top+distance {
				ev3.LCD.Set(x, y, color.Black)
			} else {
				ev3.LCD.Set(x, y, color.White)
			}
		}
	}
}

func say(text string) error {
	speak := exec.Command("espeak", "--stdout", "-a", "200", "-s", "100", text)
	play := exec.Command("aplay", "-q")
	var err error
	play.Stdin, err = speak.StdoutPipe()
	if err != nil {
		return err
	}
	err = speak.Start()
	if err != nil {
		return err
	}
	return play.Start()
}

var buttons ev3dev.ButtonPoller

func done() bool {
	b, err := buttons.Poll()
	if err != nil {
		panic(err)
	}
	if b&ev3dev.Back != 0 {
		return true
	}
	if b&ev3dev.Middle != 0 {
		must(say("Don't press that button"))
	}
	return false
}

kortschak · 2017-08-18T02:43:09Z

@dlech Can I close this?

dlech · 2017-08-18T03:00:20Z

sure.

dlech closed this as completed Aug 18, 2017

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Am I doing this right? #39

Am I doing this right? #39

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dlech commented Aug 18, 2017

Am I doing this right? #39

Am I doing this right? #39

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