Code:play 

package main

import (
	"context"
	"fmt"
	"log"

	"gocloud.dev/runtimevar"
)

func main() {
	// PRAGMA: This example is used on gocloud.dev; PRAGMA comments adjust how it is shown and can be ignored.
	// PRAGMA: On gocloud.dev, add a blank import: _ "gocloud.dev/runtimevar/constantvar"
	// PRAGMA: On gocloud.dev, hide lines until the next blank line.
	ctx := context.Background()

	// runtimevar.OpenVariable creates a *runtimevar.Variable from a URL.

	v, err := runtimevar.OpenVariable(ctx, "constant://?val=hello+world&decoder=string")
	if err != nil {
		log.Fatal(err)
	}
	defer v.Close()
	// PRAGMA: On gocloud.dev, hide the rest of the function.
	snapshot, err := v.Latest(ctx)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(snapshot.Value.(string))

	// Output
	// hello world
}

Code:play 

package main

import (
	"context"
	"fmt"
	"log"

	"gocloud.dev/runtimevar/constantvar"
)

func main() {
	// Construct a *runtimevar.Variable that always returns "hello world".
	v := constantvar.New("hello world")
	defer v.Close()

	// We can now read the current value of the variable from v.
	snapshot, err := v.Latest(context.Background())
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(snapshot.Value.(string))

}

Output:

hello world

Code:play 

package main

import (
	"context"
	"fmt"
	"log"

	"gocloud.dev/runtimevar"
	"gocloud.dev/runtimevar/constantvar"
)

func main() {
	// Construct a *runtimevar.Variable with a []byte.
	v := constantvar.NewBytes([]byte(`hello world`), runtimevar.BytesDecoder)
	defer v.Close()

	// We can now read the current value of the variable from v.
	snapshot, err := v.Latest(context.Background())
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("byte slice of length %d\n", len(snapshot.Value.([]byte)))

}

Output:

byte slice of length 11

Code:play 

package main

import (
	"context"
	"errors"
	"fmt"
	"log"

	"gocloud.dev/runtimevar/constantvar"
)

func main() {
	// Construct a runtimevar.Variable that always returns errFake.
	var errFake = errors.New("my error")
	v := constantvar.NewError(errFake)
	defer v.Close()

	// We can now use Watch to read the current value of the variable
	// from v. Note that Latest would block here since it waits for
	// a "good" value, and v will never get one.
	_, err := v.Watch(context.Background())
	if err == nil {
		log.Fatal("Expected an error!")
	}
	fmt.Println(err)

}

Output:

runtimevar (code=Unknown): my error