Monad transformers with identity monad

Question

What\'s the point in using a Monad transformer with the Identity monad rather than just using the \"standard\" version of the transformer?

Is it more flexible?

Answer 1

Back in mtl 1.0 we had both

newtype State s a = State { runState :: s -> (a, s) }

and

newtype StateT s m a = StateT { runStateT :: s -> m (a, s) }

However, this meant anybody who had to implement instances for things like MonadState wound up duplicating effort.

In transformers (and the now defunct monads-fd and monads-tf) Ross Paterson decided to use the simpler approach of only offering the latter and using Identity as the base monad.

This led to reduced implementation effort in maintaining the mtl and removed the fact that there were two different ways to implement the State monad. It did, however, make the internals of the mtl harder to teach, because you need to understand the transformers versions right out of the gate and don't get the simplified version as training wheels.

When the old mtl was retired and monads-fd became mtl 2.0, using the existing transformers this design decision was carried over.

I personally liked having the separate simple monads for pedagogical purposes at least, but there were far more people on the other side of the debate.