qualified-imports-plugin

A GHC plugin to automatically insert common qualified imports. Supports GHC 8.10 and GHC 9.

Example

This plugin, alongside with relude, allows below code to compile, without requiring explicit qualified imports.

module Main where

countChars :: Text -> [(Char, Int)]
countChars txt =
  txt
    & Text.foldl
        (\m chr -> Map.alter (Just . maybe 1 (+1)) chr m)
        Map.empty
    & Map.toList
    & sortOn (Down . snd)

main :: IO ()
main =
  countChars (Text.pack "a peck of pickled peppers")
    & mapM_ print

With a cabal stanza like:

executable example
  main-is:          Main.hs
  ghc-options:      -fplugin=QualifiedImportsPlugin
  mixins:           base hiding (Prelude)
                  , relude (Relude as Prelude)
  build-depends:    base ^>=4.15.0.0
                  , relude
                  , text
                  , containers
                  , qualified-imports-plugin
  default-language: Haskell2010

See the complete example.

It comes with (hopefully) sane defaults, but these can be extended, eg: -fplugin-opt=QualifiedImportsPlugin:Data.Graph:Graph. The defaults can also be ommitted via -fplugin-opt=QualifiedImportsPlugin:no-defaults.

Background

The best practice when using Haskell modules these days is qualified imports. This solves name clashes (without resorting to ad-hoc polymorphism), and make the code easier to follow through.

However, this also ends up introducing a bunch of import statements to most real-world modules, it is not uncommon to see sections like:

import qualified Data.Map as Map
import qualified Data.Text as Text
import qualified Data.Aeson as Aeson

This gets tedious to both read and write after a while.

This plugin is a PoC to see what would it look like if we could omit some of those.

My main use case in mind is using this alongside with the excellent relude; so, the default names reflect the naming convention there (eg. relude has LText for Data.Text.Lazy (Text) hence this plugin has import qualified Data.Text.Lazy as LText as a default).

Details

• One still needs to add required packages to the .cabal file.

• The plugin imports the defined modules, regardless if they are used or not. This is to allow the suggestion in error messages to contain the available modules:

    Not in scope: ‘Teext.pack’
    Perhaps you meant one of these:
      ‘Text.pack’ (imported from Data.Text),
      ‘LText.pack’ (imported from Data.Text.Lazy),
      ‘Text.unpack’ (imported from Data.Text)
    No module named ‘Teext’ is imported.

In the presence of orphan/overlapping instances this can break/change the meaning of code.

• It also adds the imports if an abbreviation used, but not possible to import. This again to get easier to understand errors:

    Could not load module ‘Data.Text’
    It is a member of the hidden package ‘text-1.2.4.1’.
    Perhaps you need to add ‘text’ to the build-depends in your .cabal file.
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
  |
4 | test1 = Text.pack "test1"
  |         ^^^^^^^^^