I cannot figure out why after calling ConnectToMongo variable session is still nil. I would understand it if ConnectToMongo accepts not reference type like ConnectToMongo(session mgo.Session) but reference variable type *mgo.Session must be saved after returning function ConnectToMongo

package main

import (
    "fmt"
    "gopkg.in/mgo.v2"
)

func ConnectToMongo(session *mgo.Session) {
    if session == nil {
        var err error
        session, err = mgo.Dial("localhost:27028")
        if err != nil {
            panic(err)
        }
    }
}

func main() {
    var session *mgo.Session
    ConnectToMongo(session)
    if session == nil {
        fmt.Println("nil. Why?")
    }
}

It outputs:

nil. Why?

Update

package main

import (
    "fmt"
    "gopkg.in/mgo.v2"
)

func ConnectToMongo(session **mgo.Session) {
    if session == nil {
        var err error
        *session, err = mgo.Dial("localhost:27028")
        if err != nil {
            panic(err)
        }
    }
}

func main() {
    var session *mgo.Session
    ConnectToMongo(&session)
    if session == nil {
        fmt.Println("nil. Why?")
    } else {
        fmt.Println("not nil. Ok.")
    }
}

The same output:

nil. Why?

You need to pass a pointer to pointer to store the value of the pointer. Otherwise your are copying the value of the pointer to the ConnectToMongo function.

package main

import (
    "fmt"
    "gopkg.in/mgo.v2"
)

func ConnectToMongo(session **mgo.Session) {
    if *session == nil {
        var err error
        *session, err = mgo.Dial("localhost:27028")
        if err != nil {
            panic(err)
        }
    }
}

func main() {
    var session *mgo.Session
    ConnectToMongo(&session)
    if session == nil {
        fmt.Println("nil. Why?")
    }
}