[Haskell-cafe] transformers problem: Could not deduce MonadTrans (StateT s) ??
allbery.b at gmail.com
Sat Jun 23 19:12:17 CEST 2012
On Sat, Jun 23, 2012 at 12:22 PM, Anton Kholomiov <anton.kholomiov at gmail.com
> 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 at gmail.com
wandering unix systems administrator (available) (412) 475-9364 vm/sms
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