---
title: "What does `library(<pkg>)` do?"
author: "Martin Maechler"
date: "April 2016"
output: html_document
---

### What happens when you "get" a package: `library()` etc

```
require(<pkg>) \   1) load the namespace corresponding to 'pkg'  
                \         -> *import* (*recursively*) everything needed  
                /  2) attach "the package"  to the search() path.  
library(<pkg>) /	            "the package" :=  The *export*ed objects  
```

### Every Package has two important "Meta data" files:
* `DECRIPTION`: contains `Depends:`, `Imports:`, ...
* `NAMESPACE`: has `imports()` and `exports()`
              (plus `S3methods()`, `exportClasses()`, ...)

### R functions related to this:
* `loadNamespace`
* `::`  ( and also `:::` )
*  `sessionInfo()`
*  `getAnywhere()`