import "base" System.IO

Question

In packages-haskell2010/System/IO.hs we have the following line : ``` import "base" System.IO hiding (openFile, hWaitForInput) ``` This form of `import` (where it is followed by a String and then by a module name) is not documented at Import. Am I correct in assuming that the String (in this case `"base"`) simply refers to a directory?

Accepted Answer

The string refers to a package, since this is a package-qualified import. That extension is enabled by

```
{-# LANGUAGE PackageImports #-}
```

One can use this to include the given module from the named package. In your case, it would import `System.IO` from the package "base":

With the `-XPackageImports` flag, GHC allows import declarations to be qualified by the package name that the module is intended to be imported from. For example:

```
import "network" Network.Socket
```

would import the module `Network.Socket` from the package network (any version). This may be used to disambiguate an import when the same module is available from multiple packages, or is present in both the current package being built and an external package.

In case you're wondering what packages are on your system, use `ghc-pkg list`. Note that you might need a `ghc-pkg recache` if you aborted the installation of a package in cabal, and `recache` might need administrator's rights (depending on your platform).

import "base" System.IO

Solution

Problem