What can I do to make this ghci infereced signature to compile

Vote count:

0

I have a RWST Monad, and have some generic functions to interact with the Writer part of it.

For reference, RWST is like this:

newtype RWST r w s m a = RWST {runRWST :: r -> s -> m (a, s, w)}
instance (Monoid w, Monad m) => Monad (RWST r w s m)
instance Functor m => Functor (RWST r w s m)
instance (Monoid w, MonadFix m) => MonadFix (RWST r w s m)
instance (Monoid w, MonadPlus m) => MonadPlus (RWST r w s m)
instance Monoid w => MonadTrans (RWST r w s)
instance (Monoid w, MonadIO m) => MonadIO (RWST r w s m)
instance (Monoid w, Monad m) => MonadWriter w (RWST r w s m)
instance (Monad m, Monoid w) => MonadState s (RWST r w s m)
instance (Monad m, Monoid w) => MonadReader r (RWST r w s m)
instance (Monoid w, Monad m) => MonadRWS r w s (RWST r w s m)

So I had a data Definition that is like this:

type Definition = RWS SapphireReader DefWriter DefState

Where the DefWriter is just type DefWriter = Seq Error (Error is defined by me, not the Control.Monad.Error one).

I had a working function:

tellPError :: Position -> ParseError -> Definition ()
tellPError posn err = tell (singleton $ PError posn err)

Now I need a new RWST and wish to have a generic tellPError, I tried removing the signature, loading the file in ghci and doing :t tellPError.

λ :t tellPError
tellPError :: MonadWriter (Seq Error.Error) m => Position -> ParseError -> m ()

I try giving my function that signature in the code, it won't compile:

Non type-variable argument
      in the constraint: MonadWriter (Seq Error) m
    (Use -XFlexibleContexts to permit this)
    In the type signature for `tellPError':
      tellPError :: MonadWriter (Seq Error) m =>
                    Position -> ParseError -> m ()

Do I really need this flag? I tried it and it did compile but wouldn't work.

asked 35 secs ago

chamini2

843

What can I do to make this ghci infereced signature to compile