[Haskell-cafe] MonadPeelIO instance for monad transformers on top of "forall"

Max Bolingbroke batterseapower at hotmail.com
Sat Feb 5 09:13:59 CET 2011

On 5 February 2011 02:35, Sebastian Fischer <fischer at nii.ac.jp> wrote:
> I have not used monad-peel before so please ignore my comment if I am
> missing something obvious. The documentation mentions that "Instances of
> MonadPeelIO should be constructed via MonadTransPeel, using peelIO =
> liftPeel peelIO". I think this would work at least in your simplified
> example as ReaderT is an instance of MonadTransPeel. Maybe you can take the
> same route with your actual transformer?

I did indeed try this, but the error message was even more
incomprehensible than my attempt to lift the MonadPeelIO (ReaderT ()
IO) instance directly! My reading of the error was that it suffers
from the same problem (too little polymorphism).


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