Functions / Variables of function type / Essential Go

package main

var isUserAdminFn func(string) bool

func isUserAdminProduction(userName string) bool {
    // an real implementation that talks to database
    return false
}

func isUserAdminMock(userName string) bool {
    // a fake implementation used for tests
    return userName == "admin"
}

func isUserAdmin(userName string) bool {
    return isUserAdminFn(userName)
}

func main() {
    isUserAdminFn = isUserAdminProduction
    // in test you would use:
    // isUserAdminFn = isUserAdminMock
}