Writing GHC plugin to modify AST despite failure to type-check

[Christiaan Baaij]

Another option is to use a constraint solver plugin to "tag" the locations
with a coercion, and then use a CorePlugin [1] to replace the corresponding
cast by a call to liftIO.
I've created a constraint solver plugin to tag all the locations here:
https://gist.github.com/christiaanb/5e2412bffce0fefb076d05198f94f2d8

As you can see, for:

> {-# OPTIONS_GHC -fplugin=LiftIOPlugin -ddump-ds -ddump-tc -ddump-to-file
#-}
> module Test where
>
> import Control.Monad.IO.Class
>
> program :: MonadIO m => m ()
> program = putStrLn "Hello world!"

it results in the following desugar output

> program
>   = \ (@(m_a9Ky :: * -> *)) _ [Occ=Dead] ->
>       (break<0>() putStrLn (GHC.CString.unpackCString# "Hello world!"#))
>       `cast` (Univ(representational plugin "tag_lift_io"
>                    :: IO, m_a9Ky) <()>_N
>               :: IO () ~R# m_a9Ky ())

So now you'll need to make a CorePlugin to recognize that cast and replace
it with an application with `liftIO`.
Hopefully someone else can help you with suggestions on how to conjure a
proper `liftIO` out of thin air at that point in the compiler pipeline.

[1]
https://downloads.haskell.org/ghc/9.0.1/docs/html/libraries/ghc-9.0.1/GHC-Driver-Plugins.html#t:CorePlugin

On Thu, 1 Jul 2021 at 10:24, Zubin Duggal <zubin at well-typed.com> wrote:

> You could set `-fdefer-type-errors` on the file, possibly using
> `dynflagsPlugin`. This will give your `typeCheckResultAction` an AST
> with all nodes containing type errors wrapped in an `evDelayedError`
> term. See Note [Deferring coercion errors to runtime] for more details.
> You can walk through the AST and replace these wrappers with `liftIO`
> (with the correct type and dictionary arguments) and things should
> work as you want.
>
> Of course, this will defer all type errors in the program, not just the
> ones that your plugin can solve. You could work around this by setting
> `log_action` to "upgrade" any type error warnings you didn't handle and
> arose as a result of `Reason Opt_DeferTypeErrors :: WarnReason` back to
> proper errors.
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