[Haskell-cafe] transformers problem: Could not deduce MonadTrans (StateT s) ??
Anton Kholomiov
anton.kholomiov at gmail.com
Sat Jun 23 18:22:13 CEST 2012
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)
bound by the type signature for phi :: Monad m => StateT s m ()
at FSMt.hs:28:1-22
Possible fix:
add (MonadTrans (StateT s)) to the context of
the type signature for phi :: Monad m => StateT s m ()
or add an instance declaration for (MonadTrans (StateT s))
In the expression: lift
In the expression: lift $ return ()
In an equation for `phi': phi = lift $ return ()
Failed, modules loaded: none.
Anton
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