Imp

Imp is a GHC plugin for automatically importing modules. This behavior is similar to the -fimplicit-import-qualified flag for GHCi. In short, Imp allows you to use fully-qualified identifiers without explicitly importing any modules.

This is similar to qualified-imports-plugin, but it works differently behind the scenes and supports newer versions of GHC. It is also similar to the implicit import proposal, but more limited in scope.

Basic Usage

To use Imp, add it to your package's build-depends, like this:

library
  build-depends: imp

Then you can enable it with the -fplugin=Imp flag for GHC, like this:

{-# OPTIONS_GHC -fplugin=Imp #-}
main = System.IO.print ()

For the above module, Imp will automatically import System.IO for you. It's as though you wrote this instead:

import qualified System.IO
main = System.IO.print ()

Enabling Everywhere

More often than not, you'll probably want to enable Imp for an entire component rather than for a single module. To do that, add it to your package's ghc-options, like this:

library
  ghc-options: -fplugin=Imp

Then you don't need to use the OPTIONS_GHC pragma to enable Imp.

Aliasing Modules

Sometimes you may want to refer to modules by an alias. For example you may prefer using System.IO as simply IO. Imp supports this with the --alias=SOURCE:TARGET option. Here's an example:

{-# OPTIONS_GHC
  -fplugin=Imp
  -fplugin-opt=Imp:--alias=System.IO:IO #-}
main = IO.print ()

That is the same as writing this:

import qualified System.IO as IO
main = IO.print ()

Later aliases will override earlier ones.

Aliasing Current Module

You can use the special source module name _ (a single underscore) to refer to the current module. This allows you to disambiguate identifiers without referring to the current module name. For example if you want to use This, you can do so with --alias=_:This. As a complete example, this input:

{-# OPTIONS_GHC
  -fplugin=Imp
  -fplugin-opt=Imp:--alias=_:This #-}
module Qualified.Example where
print = putStrLn . show
defaultMain = This.print ()

Will effectively produce this output:

module Qualified.Example where
print = putStrLn . show
defaultMain = Qualified.Example.print ()

Recommended Usage

Combining the previous sections, the recommended usage of Imp is to enable it in your package description (*.cabal file) along with any aliases that you want in your project. For example:

library
  build-depends: imp ^>= 1.0.0.0
  ghc-options:
    -fplugin=Imp
    -fplugin-opt=Imp:--alias=_:This
    -fplugin-opt=Imp:--alias=Data.Map.Strict:Map
    -fplugin-opt=Imp:--alias=Data.Sequence:Seq
    -fplugin-opt=Imp:--alias=Data.Set:Set
    -- and so on ...

Package Imports

It's possible that the same module name can be defined in two different packages. In normal Haskell code, you can disambiguate using the PackageImports language extension. To do the same with Imp, use the --package=MODULE:PACKAGE option. For example, consider the following module:

{-# LANGUAGE PackageImports #-}
{-# OPTIONS_GHC
  -fplugin=Imp
  -fplugin-opt=Imp:--package=Data.SemVer:semver #-}
main = print Data.SemVer.initial

That will produce the following output:

{-# LANGUAGE PackageImports #-}
import qualified "semver" Data.SemVer
main = print Data.SemVer.initial

Limitations

Due to limitations in how GHC plugins work, Imp cannot be used to automatically import modules from the same compilation unit. In typical usage this means that you cannot import modules from the same package. For example, this will not work:

-- A.hs
module A where
aThing = ()

-- B.hs
{-# OPTIONS_GHC -fplugin=Imp #-}
module B where
bThing = A.aThing

If you attempt to compile those modules in a single package, you'll get an error like this:

B.hs:3:10: error: [GHC-58427]
    attempting to use module ‘example-0:A’ (A.hs) which is not loaded
  |
3 | bThing = A.aThing
  |          ^^^^^^^^

The only workarounds are to either import the module manually or move the modules into separate packages. See issue 11 for details.

Notes

Imp operates purely syntactically. It doesn't know anything about the identifiers that you use. So if you refer to something that doesn't exist, like System.IO.undefined, you'll get an error from GHC.

Imp will never insert an import for a module that you've explicitly imported. It will only insert an import when the module is not in scope already.