Injecting imported functions using a core plugin

Josef Svenningsson josefs at
Tue Nov 26 12:55:29 UTC 2019

Yes, the plugin is fully aware of module A in my example.



From: "Domínguez, Facundo" <facundo.dominguez at>
Date: Tuesday, November 26, 2019 at 11:56 AM
To: Josef Svenningsson <josefs at>
Cc: "ghc-devs at" <ghc-devs at>
Subject: Re: Injecting imported functions using a core plugin

Hello Josef,

Do you know the location of foo when building the plugin? Otherwise, how is the plugin supposed to learn where it comes from?


On Tue, Nov 26, 2019 at 8:49 AM Josef Svenningsson <josefs at<mailto:josefs at>> wrote:
Hi ghc-devs,

I’m currently writing a core plugin that I could use some help with.
Consider the following two modules:

module A where

foo :: Int
bar :: Int

module B where

baz :: Int
baz = bar

When compiling module B I run my plugin. The goal of the plugin is to replace the occurrence of `bar` with `foo`. Note that we can be sure that `foo` is actually imported, but unfortunately doesn’t occur anywhere in B before the plugin performs the transformation.

The problem I have is that in order to inject `foo` in B I need to have an `Id` which represents `foo` and I’m having some trouble constructing such an `Id`. I’ve looked through the various environments that are available during the core to core transformations but none of them provides enough information to actually produce the `foo` `Id` as far as I can see. I hope I’m missing something. What do I need to do in order to construct the `foo` `Id` in module B?



PS. The way I’ve phrased my problem in this email it would be possible to solve it with rewrite rules. My actual use case is unfortunately more complicated and rewrite rules don’t provide enough power to do what I want.
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