Learning go language 0x08: practice using error in go language journey

* subject

https://tour.go-zh.org/methods/20

practice ： error
from Previous exercises Copy the Sqrt function , Modify it to return error value .

Sqrt When a negative number is received , A non should be returned nil The wrong value of . The plural is also not supported .
Create a new type

 type ErrNegativeSqrt float64

And for it

func (e ErrNegativeSqrt) Error() string

Method to have error value , adopt ErrNegativeSqrt(-2).Error() Calling this method should return "cannot Sqrt negative number: -2".

Be careful : stay Error Method fmt.Sprint(e) It's going to put the program in a dead cycle . You can do this by first converting e To avoid this problem ：fmt.Sprint(float64(e)). Why is that ？

modify Sqrt function , To accept a negative number , return ErrNegativeSqrt value .

* Code

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) {
    
	if x < 0 {
    
		return 0, ErrNegativeSqrt(x)
	}
	z := 1.0
	for i := 1; i < 10; i++ {
    
		z -= (z*z - x) / (2*z)
	}
	return z, nil
}

func main() {
    
	fmt.Println(Sqrt(2))
	fmt.Println(Sqrt(-2))
}

Output ：

1.4142135623730951 <nil>
0 cannot Sqrt negative number: -2

* take fmt.Sprint(float64(e)) Change it to fmt.Sprint(e), The stack will overflow

If hold fmt.Sprintf("cannot Sqrt negative number: %v", float64(e)) Change to fmt.Sprintf("cannot Sqrt negative number: %v", e), The running result is ：

1.4142135623730951 <nil>
runtime: goroutine stack exceeds 250000000-byte limit
fatal error: stack overflow

runtime stack:
runtime.throw(0x1054d9, 0xe)
	/usr/local/go/src/runtime/panic.go:617 +0x80
runtime.newstack()
	/usr/local/go/src/runtime/stack.go:1041 +0x960
runtime.morestack()
	/usr/local/go/src/runtime/asm_amd64p32.s:299 +0xc0

goroutine 1 [running]:
sync.(*Mutex).Lock(0x3422094, 0x0)
	/usr/local/go/src/sync/mutex.go:72 +0x3a0 fp=0x8c00360 sp=0x8c00358 pc=0x99b80
sync.(*Pool).getSlow(0x1ad078, 0x8, 0x3422080, 0x0)
	/usr/local/go/src/sync/pool.go:165 +0xe0 fp=0x8c00390 sp=0x8c00360 pc=0x9a400
sync.(*Pool).Get(0x1ad078, 0x1b4540, 0xfefc0004, 0x0)
	/usr/local/go/src/sync/pool.go:141 +0x1a0 fp=0x8c003b8 sp=0x8c00390 pc=0x9a2a0
fmt.newPrinter(0x8c003f0, 0x0)
	/usr/local/go/src/fmt/print.go:134 +0x40 fp=0x8c003d0 sp=0x8c003b8 pc=0xce6e0
fmt.Sprintf(0x1082ef, 0x1f, 0x8c00420, 0x1, 0x1, 0x8c00438, 0x18, 0x139f)
	/usr/local/go/src/fmt/print.go:213 +0x20 fp=0x8c00400 sp=0x8c003d0 pc=0xcec40
main.ErrNegativeSqrt.Error(...)
	/tmp/sandbox804641635/main.go:10
main.(*ErrNegativeSqrt).Error(0x67a3330, 0x122ecc, 0x67c7300, 0xf1b40)
	<autogenerated>:1 +0xc0 fp=0x8c00430 sp=0x8c00400 pc=0xd97c0
fmt.(*pp).handleMethods(0x67c7300, 0x76, 0x6cd01, 0x139f)
	/usr/local/go/src/fmt/print.go:610 +0x1e0 fp=0x8c00480 sp=0x8c00430 pc=0xd2480
fmt.(*pp).printArg(0x67c7300, 0xf1b40, 0x67a3330, 0x76)
	/usr/local/go/src/fmt/print.go:699 +0x260 fp=0x8c004d0 sp=0x8c00480 pc=0xd2d00
fmt.(*pp).doPrintf(0x67c7300, 0x1082ef, 0x1f, 0x8c00598, 0x1, 0x1)
	/usr/local/go/src/fmt/print.go:1016 +0x180 fp=0x8c00548 sp=0x8c004d0 pc=0xd7640
fmt.Sprintf(0x1082ef, 0x1f, 0x8c00598, 0x1, 0x1, 0x8c005b0, 0x18, 0x139f)
	/usr/local/go/src/fmt/print.go:214 +0x60 fp=0x8c00578 sp=0x8c00548 pc=0xcec80
main.ErrNegativeSqrt.Error(...)
	/tmp/sandbox804641635/main.go:10

 A little  （ There are still a lot of it log）

An understanding of code looping ：
Code in question , as follows ：

func (e ErrNegativeSqrt) Error() string {
	return fmt.Sprintf("cannot Sqrt negative number: %v", e)
}

First execution ErrNegativeSqrt Of Error() function , By main Function fmt.Println(Sqrt(-2)) The trigger .Sqrt(-2) return error by ErrNegativeSqrt, stay main Print this in error, Will execute ErrNegativeSqrt Of Error() function .
And then in Error() Internal function , perform fmt.Sprintf("cannot Sqrt negative number: %v", e),e by ErrNegativeSqrt , therefore , Print again ErrNegativeSqrt, therefore , It also calls ErrNegativeSqrt Of Error() function . So there's a dead cycle , Until the call stack overflows and an error is reported .