tour: unclear instruction for errors exercise

[adderek]

Context: https://tour.golang.org/methods/20

The instruction does not clearly define what to do.
I was expecting an output:

1.41
cannot Sqrt negative number: -2

while the only output I was able to produce is

1.41 <nil>
0 cannot Sqrt negative number: -2

as fmt.Println(Sqrt(2)) will print "error"
and fmt.Println(Sqrt(-2)) will print the float64 part

It should be also noted earlier that %g is the way to print float64

My code:

type ErrNegativeSqrt float64
func (e ErrNegativeSqrt) Error() string {
	return fmt.Sprintf("cannot Sqrt negative number: %g", float64(e))
}
func (e ErrNegativeSqrt) String() string {
	return fmt.Sprintf("%f",e)
}
func Sqrt(x float64) /*ErrNegativeSqrt*/(float64, error) {
	if x<0 {
		return 0,ErrNegativeSqrt(x) // Why do I need to include 0? nil seems more appropriate
	}
	var z1, z2 float64 = 1,0
	for i:=0; i<8; i++ {
		z2 = z1-((z1*z1-x)/(2*z1))
		z1=z2
	}
	return z2, nil
}

[mvdan]

I was expecting an output:
1.41
cannot Sqrt negative number: -2

Why? If you're printing the result of the call, you'll get two elements.

[adderek]

Why? If you're printing the result of the call, you'll get two elements.

Because expected output was not clearly defined and sqrt(2) feels just right to return a single (might be complex) value. Remember that the tour is meant for newbies.
Please, just define bounding conditions: what is the input, what is expected output.
Also look at issues #230 and #59

[paulgrove]

👍 was unclear to me also what the exercise was intending.

Got there in the end, had to search internet (found this) when trying to return nil, ErrNegativeSqrt(x)

More focus on the standard return pattern for errors in the previous step with a full working implementation would have been appreciated.