[Haskell-cafe] MTL vs Free-monads, what are your experiences

[Matt]

The mtl technique subsumes the free monad technique. if you have a term:

getCurrentTime :: MonadClock m => m UTCTime

Then you can *use* that function as anything that satisfies the constraint.
Given an `IO` instance, that can just get the current time: `getCurrentTime
:: IO UTCTime`. Given a mock instance, that can be `getCurrentTime ::
MockClock UTCTime`. Given an instance on Free, you'd have `getCurrentTime
:: Free CurrentTimeF UTCTime`

I generally find it more pleasant to write functions in mtl style. If
you're after more concrete guarantees on the DSL you're building and see
yourself doing a lot of introspection and optimization, then a Free monad
approach fits the bill.

Matt Parsons

On Fri, Oct 14, 2016 at 11:35 AM, Damian Nadales <damian.nadales at gmail.com>
wrote:

> Hi,
>
> I was looking into free monads for designing a DSL for describing
> scenarios of the form:
>   scenario = do
>     aId <- createA
>     b0Id <- createB id
>     b1Id <- createB id
>     link b0 b1
>
> In our company we use a graph database, and currently we're setting up
> the test data using raw queries O.O So I wanted to come up with a
> better abstraction, and also enable us to do property based testing
> (by testing on random scenarios like the one above).
>
> Anyway, I ran into this presentation:
>     http://www.slideshare.net/jdegoes/mtl-versus-free
>     http://degoes.net/articles/modern-fp-part-2
>
> In which monad transformers and free monads are compared. Do you have
> any experience using any of these approaches. If so would you mind
> sharing? ;)
>
> Thanks!
> Damian
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