glob-imports

A Haskell source preprocessor which generalizes persistent-discover and hspec-discover, allowing you to metaprogram with source files.

A big limitation of TemplateHaskell is that it cannot modify the import list of a file. The only way to metaprogram with the imports is to use a source preprocessor like this.

As an executable

Let's say you've got a ton of persistent database models, all defined in a module hierarchy like this:

src/
  Models/
    Foo.hs
    Bar.hs
    Baz.hs
    Blargh.hs
    What.hs
    OhNo.hs

If you're using persistent to automatically generate migrations, you'll want to have all the [EntityDef] in scope from each module. You can do that by importing each module that defines models and calling $(discoverEntities), introduced in persistent-2.13.

But you may forget to import a module.

This utility splices in a qualified import statement for all the modules defined in the current directory and sub-directories, along with a list of imported modules. From this, you can use discover-instances or other TemplateHaskell code to work with these.

To use it, place the following command in a file, located in the same directory that your Haskell modules are in:

-- src/Models/All.hs

{-# OPTIONS_GHC -F -pgmF glob-imports #-}

module Models.All where

import Database.Persist.Sql
-- GLOB_IMPORTS_SPLICE

allEntities :: [EntityDef]
allEntities = $(discoverEntities)

The preprocessor will glob files in the current directory, and import all of them qualified. The result would look like this:

-- src/Models/All.hs

-- post-processed

module Models.All where

import Database.Persist.Sql
import qualified Models.Foo
import qualified Models.Bar
import qualified Models.Baz
import qualified Models.Blargh
import qualified Models.What
import qualified Models.OhNo

_importedModules :: [String]
_importedModules =
    [ "Models.Foo"
    , "Models.Bar"
    , "Models.Baz"
    , "Models.Blargh"
    , "Models.What"
    , "Models.OhNo"
    ]

allEntities :: [EntityDef]
allEntities = $(discoverEntities)

You'll need to add glob-imports to the build-tool-depends on your cabal file for this to work.