An issue with this approach is that it fails if you have a concrete
monad instead of an mtl-style function.
For example, with
newtype MyIO a = MyIO (IO a)
deriving newtype (Functor, Applicative, Monad, MonadIO)
program :: MyIO ()
program = putStrLn "Hello world!"
GHC will reject the program because it can't unify `IO` and `MyIO`
before it can even get to the constraint solver plugin.
In general, implementing a plugin like this is a nice way to understand
and familiarise yourself with plugins and the GHC API, but for practical
purposes it would be best to use something like the `lifted-base` or
`unliftio` libraries to access lifted version of common IO operations.
On 21/07/01 10:54, Christiaan Baaij wrote:
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 <zu...@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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