Re: [Haskell-cafe] transformers problem: Could not deduce MonadTrans (StateT s) ??

On Sat, Jun 23, 2012 at 12:22 PM, Anton Kholomiov

wrote:

Why this function doesn't compile?

phi :: Monad m => StateT s m () phi = lift $ return ()

I get (ghc-7.4.1)

Could not deduce (MonadTrans (StateT s)) arising from a use of `lift' from the context (Monad m)

This means exactly what it says: you have stated that m must be a Monad, but you didn't say anything about whether it's a MonadTrans, so you can't use "lift". The correct signature would be phi :: (Monad m, MonadTrans m) => StateT s m () -- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms