Documentation

Laws

tell mempty ≡ pure ()

tell (a <> b) ≡ tell a *> tell b

How does this relate to MonadWriter?

MonadTell is a generalisation of MonadWriter. It only provides tell; a function that 'appends' a monoidal value to some output. Morally, we have class MonadTell w m => MonadWriter w m where .... See WrappedMonadWriter for the witness of this.

MonadWriter's listen and pass functions require the monad to hold onto the output for an arbitrarily long time. This can cause applications to use memory linear in the number of tells, when constant memory usage would suffice.

A motivating example is writing monoidal results to a file. Using MonadWriter (via a WriterT), you would have to accumulate the the entire output and then write it to the file. In contrast, MonadTell allows you to write each result to the file as it's obtained, allowing the result to be freed while the rest of the program runs.

A proof that a MonadWriter instance implies a MonadTell instance.