This question already has answers here:
What does an underscore and interface name after keyword var mean?
(2 answers)
Closed 5 years ago.
I was reading DigitalOcean's golang client. I noticed that they create an instance of their *Op struct in a _ variable. Example:
https://github.com/digitalocean/godo/blob/master/droplets.go#L32
var _ DropletsService = &DropletsServiceOp{}
Why is this line needed?
This line is a compile time check that *DropletsServiceOp satisfies the DropletsService interface.
The line has no effect on the execution of the program.
If you look at the blame on that file, at that line, you get a clue:
https://github.com/digitalocean/godo/blame/master/droplets.go#L32
It does a compile-time check that *DropletsServiceOp satisfies the DropletsService interface. Prior to the commit introducing that, they were doing this in their test suite.
Related
This question already has an answer here:
Statement blocks in go
(1 answer)
Closed 7 months ago.
when writing gin(a go web frame) code, I found a
piece of code like this:
r := gin.New()
apiv1 := r.Group("/api/v1")
{ // don't understand
apiv1.GET("/tags", v1.GetTags)
apiv1.POST("/tags", v1.AddTag)
}
It's ok and have no warn or error.
But I do don't know what's the newline braces mean, or it just has no effect?
From Go spec docs
blocks nest and influence scoping
This question already has answers here:
What is "_," (underscore comma) in a Go declaration?
(9 answers)
Closed 7 years ago.
I'm new to Go and came across this line of code while browsing through some other threads:
if _, err := os.Stat("/path/to/whatever"); os.IsNotExist(err)
What does the _, after the if mean? Is it specifying that something will be assigned in the if condition (as it appears is happening with err)? I couldn't find an example of this syntax on the wiki and I'm very curious to see what it's used for.
Here's a link to the thread I was looking at if it helps:
How to check if a file exists in Go?
Because os.Stat returns two values, you have to have somewhere to receive those if you want any of them. The _ is a placeholder that essentially means "I don't care about this particular return value." Here, we only care to check the error, but don't need to do anything with the actual FileInfo Stat gives us.
The compiler will just throw that value away.
This question already has answers here:
Multiple values in single-value context
(6 answers)
Closed 7 years ago.
Is there any way in Go to do this:
segment := Segment{
CumulativeDistanceMm: strconv.Atoi(record[9]),
Length: strconv.Atoi(record[1]),
LinkId: strconv.Atoi(record[8]),
SegmentId: strconv.Atoi(record[2]),
}
The error that I get is that strconv.Atoi returns multiple values so I can't assign it directly to the struct properties. If it was a variable I could use the underscore to ignore the second value. Can I do something similar for structs?
strconv.Atoi can fail and you have to deal with this failure. If such failures are absolutely impossible you would write a function func MustAtoi(s string) int which panics on failure and use that one in your struct initialization.
In Go doing some programming instead of using syntactical sugar or fancy syntax is common.
Most probably you should rethink your error handling.
This question already has answers here:
F# Functions vs. Values
(4 answers)
Closed 7 years ago.
I have F# code something like this:
let ran = new System.Random()
let makeVal = ran.NextDouble()
Why when I use makeVal do I get the same random number on every call within one run of the app session (i.e. it's not a seed issue).
Values in F# are immutable by default. So makeVal will not change after the first binding. To get different random values you should call ran.NextDouble() again.
For example use the function:
let makeVal() = ran.NextDouble()
This question already has an answer here:
naked asterisk as parameter in method definition: def f(*)
(1 answer)
Closed 10 years ago.
I was poking through the Rails 3 ActiveRecord source code today and found a method where the entire parameter list was a single asterisk.
def save(*)
I couldn't find a good description of what this does (though I have some ideas based on what I know about splat arguments).
What does it do, and why would you use it?
It means it can have any number of arguments (including zero) and it discards all those arguments.