How to use custom packages

Question

I'm trying to create and use a custom package in Go. It's probably something very obvious but I cannot find much information about this. Basically, I have these two files in the same folder:

mylib.go

package mylib

type SomeType struct {

}

main.go

package main

import (
    "mylib"
)

func main() {

}

When I try to go run main.go, I get this error:

main.go:4:2: import "mylib": cannot find package

I've tried to run go build mylib.go first but it doesn't seem to be doing anything (no file generated, no error message). So any idea how I could do this?

Solution 1

First, be sure to read and understand the "How to write Go code" document.

The actual answer depends on the nature of your "custom package".

If it's intended to be of general use, consider employing the so-called "Github code layout". Basically, you make your library a separate go get-table project.

If your library is for internal use, you could go like this:

1. Place the directory with library files under the directory of your project.
2. In the rest of your project, refer to the library using its path relative to the root of your workspace containing the project.

To demonstrate:

src/
  myproject/
    mylib/
      mylib.go
      ...
    main.go

Now, in the top-level main.go, you could import "myproject/mylib" and it would work OK.

Solution 2

For this kind of folder structure:

main.go
mylib/
  mylib.go

The simplest way is to use this:

import (
    "./mylib"
)

Solution 3

I am an experienced programmer, but, quite new into Go world ! And I confess I've faced few difficulties to understand Go... I faced this same problem when trying to organize my go files in sub-folders. The way I did it :

GO_Directory ( the one assigned to $GOPATH )

GO_Directory //the one assigned to $GOPATH
__MyProject
_____ main.go
_____ Entites
_____ Fiboo // in my case, fiboo is a database name
_________ Client.go // in my case, Client is a table name

On File MyProject\Entities\Fiboo\Client.go

package Fiboo

type Client struct{
    ID int
    name string
}

on file MyProject\main.go

package main

import(
    Fiboo "./Entity/Fiboo" 
)

var TableClient  Fiboo.Client

func main(){
    TableClient.ID = 1
    TableClient.name = 'Hugo'

    // do your things here
}

( I am running Go 1.9 on Ubuntu 16.04 )

And remember guys, I am newbie on Go. If what I am doing is bad practice, let me know !

Solution 4

For a project hosted on GitHub, here's what people usually do：

github.com/
  laike9m/
    myproject/
      mylib/
        mylib.go
        ...
      main.go

mylib.go

package mylib

...

main.go

import "github.com/laike9m/myproject/mylib"

...

Solution 5

I try so many ways but the best I use go.mod and put

module nameofProject.com

and then i import from same project I use

import("nameofProject.com/folder")

It's very useful to create project in any place

View more solutions

Comments

[laurent]

I didn't set a GOPATH variable, only GOROOT.

[Joe]

run go install under the directory of mylib first, and try again.

[kostix]

See also this thread.

[hunter_tech]

@Joe, it still can't work by running "go install" inside

[nemo]

This does not work anymore in recent versions of go as the package will not be found. The correct import would be foo/mylib (assuming the folder containing main.go is foo).

[laurent]

@nemo, with the latest version of go, I always use "./mylib" and it works.

[canadadry]

Using go 1.2 and I agree with @this.lau_

[photoionized]

Be aware that this makes go install break. If you're building a standalone project that you want people to download and run go build on, this is fine--however, I would employ the "Github code layout" mentioned above (even if off bitbucket, or similar) if you want full go install support.

[Kiril]

If I create a new project (myproject2) under src/, how could I import mylib?

[kostix]

@Kiril, you mean, how do you import mylib in the code of myproject2? Then the answer is "by using import "myproject/mylib" -- the idea is that Go searches for imported paths under each directory it extracts from the GOPATH environment variable (they are called "workspaces"), but this search is (luckily) not recursive, so such paths are effectively "anchored" at their respective workspaces.

[kostix]

Another must-have read: "Package names".

[Gustav]

this will not work for go gettable paths such as github hosted packages.

[King Jk]

I suggestionDo not use this way. It's will breaking godef. godef it does not understand about "." imports

[cmcginty]

This answer is ambiguous in that there are two mylib items listed and it's not clear if this is a requirement or just a simplification of the example. If you could clarify in the answer it would be more useful.

[Matthias Sommer]

I have several microservices which all depend on the same mylib. If I change mylib, I would have to change it in all services. How would you handle this?

[kostix]

@MatthiasSommer, typically—by extracting that mylib into a common package each microservice uses. Exactly how "uses" is defined, depends on your preferred workflow. In enterprise-grade development, vendoring is typically used, but with the recent go mod developments, a module might be the answer (and go mod supports vendoring of modules as well).

[Lei Yang]

so the package to be imported can only be .go file? how to build package to .lib file then import the lib file?

[kostix]

@LeiYang, "the package to be imported" is a directory with one or more Go source files. As to "how to build package to a .lib file" and then import the lib file"—the answer depends on what you really intended to ask for. If you're concerned with compilation speed, then fear not: the Go toolchain caches all build results on a per-package basis. If, instead, you wanted to ask whether it's possible to compile and distribute a binary-only compiled package, then the answer is no.

[kostix]

@LeiYang, but note that if for some reason you still desperately need something like this, on some platforms Go supports plugins.

[Admin]

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[Subhashis Pandey]

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