Go boss realizes his conscience and is willing to clear the map?

The same thing was discussed back and forth, closed and opened again, new ones were created after being closed, new ones were banned, new arguments were discovered, and then new ones were created. Open new one. This is very common in the workplace, and it also appears in Go proposal discussions...

What I want to share today is a controversy over NaN in Go map and the possible upcoming API increase.

Background and test questions

What is NaN

In computer science, there is a magic The value is called: NaN (Not a Number, not a number). It is a class of values ​​of numeric data type that represents undefined or unrepresentable values. Often used in floating point operations. NaN was first introduced in the 1985 IEEE 754 floating-point standard.

There will be problems when storing and comparing NaN values. Because when judging whether a value is NaN, the comparison cannot be made by judging x=NaN or x≠NaN. But because NaN is never equal to itself, you can judge whether x is a NaN value by judging x=x or x≠x, which will return False and True respectively.

When NaN is compared with another floating point number x (where x can be a normal value, plus or minus infinity, or NaN), the comparison results are as follows:

##NaN ≠ xTrue

The understanding here is very important for the following test questions.

Go IEEE-754 exam questions

I saw a question about IEEE-754 floating point numbers on Go101. I will quote the question here, as follows What does the code output?

The following code:

package main

import "math"

func main() {
 a, b, c := 2.0, 1.0, 0.0
 x, y := a/c, b/c     // infinity
 n := math.NaN()      // not a number
 m := math.Sqrt(-1.0) // not a number
 println(x == y, m == n)
}

• A: true true.
• B: true false.
• C: false false.
• D: false true.

What is the answer? Is it A or D?

Analyze the above program. The variables x and y are Inf positive infinity. m, n are NaN infinite values.

The correct answer is: B.

Did you get the answer right?

Proposal

After having a basic understanding of NaN, we can officially enter the topic. In Go map association proposals, new APIs are often mentioned to meet the demands of clearing the map:

But in many discussions, the official Go team gave The solution is:

for k := range m {
    delete(m, k)
}

and closed the related proposal, ending this issue. We, the workers, are left with doubts on our faces like “Okay, this will work.” This is the background of this type of proposal.

But there is a pitfall in this. When it contains any NaN key value, the map will not be cleared by loop delete. Once there is NaN in your map, but you for delete again, thinking that it was deleted, but in fact it was not, it will produce a leak-like effect.

Therefore, Russ Cox, the soul of the Go team, re-initiated a new proposal "proposal: spec: add delete(m) to clear map^[1]". As shown below:

#I hope to use this to solve the problem of map in NaN, and at the same time complete the community demands such as map clearing/resetting/cleaning that have been discussed.

In other words, the following features are newly added:

delete(m)

Supports the function of clearing map (even if it contains NaN).

Summary

There are several voices regarding this proposal, including naming it as evil, and having different opinions on whether to call it delete or clear.

// Clear removes all entries from m, leaving it empty.
func Clear[M ~map[K]V, K comparable, V any](m M "M ~map[K]V, K comparable, V any")

delete(m)

也有声音提到不允许引入 NaN 值，但显然。在 Go1 已经很难了，因为 NaN 已经被允许引入，球已经在锅里了。

对于 map 新增 API 用于清空/重置/清除的作用，你怎么看呢？还是说你也更喜欢对 NaN 单独的处理？例如 panic？

Comparison	Result
NaN ≥ x	False
NaN ≤ x	False
NaN > x	False
NaN	False
NaN = x	False

The above is the detailed content of Go boss realizes his conscience and is willing to clear the map?. For more information, please follow other related articles on the PHP Chinese website!