proposal: encoding/json: Add method MustFloat64 and MustInt64 for json.Number

[MakDon]

Proposal Details

It would be more convinced if we could have methods MustFloat64 and MustInt64 that wraps Float64 and Int64.

Why

Assume we need to copy some field from a json object to another struct:

type Src struct {
	Foo json.Number `json:"foo"`
}

type Dest struct {
	Foo int `json:"foo"`
	//Other fields
}

func loadSrc() Src {
	return Src{Foo: "100"}
}

func getDest() {
	src := loadSrc()
	tmpNum, _ := src.Foo.Int64()
	dest := &Dest{
		Foo: int(tmpNum),
		// Other fields
	}
	
}

A tmpNum is necessary to receive the returned value from Int64(). But if we have a MustInt64, we could do the same thing like:

func getDest() {
	src := loadSrc()
	dest := &Dest{
		Foo: int(src.Foo.MustInt64()),
		// Other fields
	}
}

If the dest has multi fields that comes from different json.Number, we need to declare multi tmp variables for each field.

And such expression is not allowed because Dest.Foo is int but Int64() returns a Int64

dest.Foo, _ = src.Foo.Int64()

With MustInt64, this expression is valid:

dest.Foo = int(src.Foo.MustInt64())

How

A simple wrapper for Float64 and Int64:

func (n Number) MustFloat64() float64 {
	f, err := strconv.ParseFloat(string(n), 64)
	if err != nil {
		panic(err)
	}
	return f
}

func (n Number) MustInt64() int64 {
	i, err := strconv.ParseInt(string(n), 10, 64)
	if err != nil {
		panic(err)
	}
	return i
}

Question

• If we should panic if this field is empty string?
• panic or return 0 when ParseInt/ParseFloat failed?

[gabyhelp]

Related Issues and Documentation

• encoding/json: should try to convert strings<->numbers without errors if it's possible #22463 (closed)

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[AlexanderYastrebov]

See related #54297 that proposes generic helper.

[seankhliao]

I think this should fold into #54297
Panic is not a general purpose error handler, and the existing must variants are intended for global values that the programmer knows cannot fail. User provided json does not fall into that category.