š A Tour of Go : Exercise: Errors
Like Interfaces, the Tour didnāt really do it for me on Errors either. Too absract, and not enough explanation of the code examples for my liking. It also doesnāt cover the errors package which other tutorial do. Iām not clear if thatās because the errors package isnāt used much, or the Tour focusses only on teaching the raw basics.
Iām quickly learning to head to gobyexample.com each time for more reference on things that arenāt making sense (along with https://www.calhoun.io/ too). The errors page on GoByExample is a good one, and I like how it links through to the Go Playground with each example. The Go Blogās Error handling and Go is also a good reference, and this blog has some recent updates for Go 1.13.
Once Iād gone through the additional links the errors exercise was OK to figure out:
package main
import (
"fmt"
)
type ErrNegativeSqrt float64
func (e ErrNegativeSqrt) Error() string {
return fmt.Sprintf("cannot Sqrt negative number: %v", float64(e))
}
func Sqrt(x float64) (float64, error) {
fmt.Printf("\n--\nSqrt called with value: %v\n", x)
if x < 0 {
fmt.Printf("\t%v is less than zero. Returning with error.\n", x)
return 0, ErrNegativeSqrt(x)
}
guess := 1.0
limit := 10
for i := 0; i < limit; i++ {
adj := (guess*guess - x) / (2 * guess)
if result := guess * guess; result == x {
fmt.Printf("\tā
Guess %d is correct:\t%g\n", i, guess)
return guess, nil
} else if result > x {
fmt.Printf("\tšŗGuess %d is too high:\t%g\n", i, guess)
guess -= adj
} else {
fmt.Printf("\tš»Guess %d is too low:\t%g\n", i, guess)
guess -= adj
}
}
return guess, nil
}
func main() {
for _, x := range []float64{-9, 9} {
if result, ok := Sqrt(x); ok == nil {
fmt.Printf("-> result: %v\n", result)
} else {
fmt.Printf("** ERROR %v",ok.Error())
}
}
}
--
Sqrt called with value: -9
-9 is less than zero. Returning with error.
** ERROR cannot Sqrt negative number: -9
--
Sqrt called with value: 9
š»Guess 0 is too low: 1
šŗGuess 1 is too high: 5
šŗGuess 2 is too high: 3.4
šŗGuess 3 is too high: 3.023529411764706
šŗGuess 4 is too high: 3.00009155413138
šŗGuess 5 is too high: 3.000000001396984
ā
Guess 6 is correct: 3
-> result: 3
Try it out: https://play.golang.org/p/mLa5RqwYckb
This bit had me puzzled:
Note: A call to
fmt.Sprint(e)inside theErrormethod will send the program into an infinite loop. You can avoid this by convertingefirst:fmt.Sprint(float64(e)). Why?
If I changed it to
func (e ErrNegativeSqrt) Error() string {
return fmt.Sprintf("cannot Sqrt negative number: %v",e)
}
then running it in the Tour window failed (as expected)
--
Sqrt called with value: -9
-9 is less than zero. Returning with error.
Program exited: status 2.
Running it in VSCode gave another error:
runtime: goroutine stack exceeds 1000000000-byte limit
fatal error: stack overflow
and sticking a print debug into the function shows that itās recursively called:
func (e ErrNegativeSqrt) Error() string {
fmt.Println("ErrNegativeSqrt.Error")
return fmt.Sprintf("cannot Sqrt negative number: %v",e)
}
--
Sqrt called with value: -9
-9 is less than zero. Returning with error.
ErrNegativeSqrt.Error
ErrNegativeSqrt.Error
ErrNegativeSqrt.Error
ErrNegativeSqrt.Error
ErrNegativeSqrt.Error
ErrNegativeSqrt.Error
[ā¦]
Butā¦ I donāt understand why. StackOverflow turns up this explanation
fmt.Sprint(e)will calle.Error()to convert the valueeto a string. If the Error() method callsfmt.Sprint(e), then the program recurses until out of memory.You can break the recursion by converting the e to a value without a String or Error method.
Thus e is converted to float64:
func (e ErrNegativeSqrt) Error() string {
return fmt.Sprintf("cannot Sqrt negative number: %v", float64(e))
}
This comment offers a neat alternative too:
Isnāt it enough to convert e to a type which may have a String/Error method that doesnāt recurse infinitely?
And hence specifying a different verb works:
func (e ErrNegativeSqrt) Error() string {
return fmt.Sprintf("cannot Sqrt negative number: %f",e)
}
--
Sqrt called with value: -9
-9 is less than zero. Returning with error.
** ERROR cannot Sqrt negative number: -9.000000
Top comments (2)
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What the hell?