[Haskell-cafe] Lifting Writer to ListT

Nicola Gigante nicola.gigante at gmail.com
Fri Mar 6 10:23:02 UTC 2015

> Il giorno 03/mar/2015, alle ore 17:22, Arjun Comar <nrujac at gmail.com> ha scritto:
> You might prefer using the ListT <https://hackage.haskell.org/package/pipes-4.1.4/docs/Pipes.html#g:5> implementation from the Pipes package which has instances for MonadReader, MonadWriter, and MonadState already. Also, the provided source might give you some insight into how to write these instances for the list-t:ListT type.

That sounds interesting, but since I don’t know the Pipes ecosystem
I don’t understand how to extract the values produced by a ListT action,
because runListT returns m () instead of m [a] or something like that.

I understand ListT is a “Producer” but then how to I extract into a list
the values produced by a pipes Producer?


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