Question

While looking through the transformers package, I found this monad transformer called IdentityT.

Although I understand how the Identity monad is used (e.g. State is just an alias for StateT Identity) and how monad transformers work in general, I have no idea how that relates to IdentityT.

Since it’s not in the MTL, I’m guessing it was added in there just for completeness and has no practical use. Is that correct?

Answer 1

Well the linked documentation does say

This is useful for functions parameterized by a monad transformer.

Though I’m not aware of any situations where this is actually the case. Theoretically if you have a function like foo :: (MonadTrans t, Monad m) => t m a -> b for some useful b, then you might want to be able to “dumb it down” to essentially m a -> b by using t = IdentityT.

But IdentityT is to MonadTrans what Identity is to Monad. It is the “pass-through” transformer, as Identity is the “pass-through” monad. Just check out the source; it’s rather simple. IdentityT SomeMonad a should behave identically to SomeMonad a, the only difference being the presence of an extra newtype (which, of course, is removed at compile time)