09_conversion.md

Value Conversions

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• Overview
• String to Number
• String and Number to Boolean
• Difference between GetString and MustGet(...).String()

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Overview

In encoding/json, there is a tag called "string", which means converting the current value (no matter what type it is) to a JSON string. For example:

type st struct {
    Num int `json:"num,string"`
}

func main() {
    st := st{Num: 12345}
    b, _ := json.Marshal(&st)
    fmt.Println(string(b))
	// Output: {"num":"12345"}
}

It is clear that num is serialized as a string instead of a number.

Jsonvalue also supports reading this type of value from a string. Please read the following descriptions for details.

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String to Number

All number-typed GetXxx methods support parsing a number from a string value.

However, if the target value is not a number-typed JSON value, the error will not be nil. Various types of errors may be returned due to different situations:

• If the target key is not found, returns ErrNotFound
• If the target value is a number, the number type will be returned with a nil err.
• If the target value is neither a string nor a number, returns ErrTypeNotMatch
• If the target value is a string, then parse the value of the string, and:
  • if the parsing succeeds, returns the number with error ErrTypeNotMatch
  • if the parsing fails (illegal number), returns error ErrParseNumberFromString, including the detailed parsing description.

You can use errors.Is() to check the different types of errors. For example:

	const raw = `{"legal":"1.25E-1","illegal":"ABCD"}`
	v := jsonvalue.MustUnmarshalString(raw)

	f, err := v.GetFloat64("legal")
	fmt.Println("01 - float:", f)
	fmt.Println("01 - err:", err)

	f, err = v.GetFloat64("illegal")
	fmt.Println("02 - float:", f)
	fmt.Println("02 - err:", err)

Output:

01 - float: 0.125
01 - err: not match given type
02 - float: 0
02 - err: failed to parse number from string: parsing number at index 0: zero string

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String and Number to Boolean

Similar to numbers, strings can hold a boolean value.

A true will be returned in only one situation when converting a string to boolean:

• The value of the string is exactly the lower-cased string "true".

In other cases, false will be returned for the conversion.

Numbers can also be converted into boolean values. As long as the number value does NOT equal zero, true will be returned. Otherwise, false.

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Difference between GetString and MustGet(...).String()

The logic between GetXxx() series methods and their corresponding MustGet().Xxx() functions is mostly the same. However, the Xxx() methods do not return an error.

For example:

	const raw = `{"legal":"1.25E-1","illegal":"ABCD"}`
	v := jsonvalue.MustUnmarshalString(raw)
	f := v.MustGet("legal").Float64()
	fmt.Println("float:", f)
	// Output: float: 0.125

But there is one exception: GetString and MustGet().String()

GetString returns a string of a string-typed JSON value. If the target is not found, or is not a string, an error will be returned.

But String is quite special. On one hand, it can get the string value of a string-typed JSON value. On the other hand, it implements the fmt.Stringer interface, which is invoked by the %v keyword of the fmt package.

Therefore, the logic of String is a bit complicated:

• If the current *jsonvalue.V is string-typed, returns its string value.
• If number-typed, returns the string representation of the number.
• If null-typed ("null" type defined in JSON standard), returns null.
• If boolean-typed, returns true or false.
• If object-typed, returns a string with the format {K1: V1, K2: V2}, with no escaping, not in JSON format.
• If array-typed, returns a string with the format [v1, v2]. Also, no escaping.