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

[Richard Wallace]

I would call the approach in quine a lensy-mtl style. It's ok as far as it
goes, but since you are using concrete environment values it isn't great if
you want to do testing of things like database code without having a "real"
backend hooked up. The typical approach then is to create your own
type-class and instances

  class MyBackend where ...

  instance (MonadReader r m, HasDb r) => MyBackend m where ...

  instance (MonadState s m, HasTestState s) => MyBackend m where ...

Of course, now our problem is that our module with this abstraction depends
on the module with the db and the test state. Unless we create orphan
instances, which I prefer to avoid. This is one area where I like the Free
monad approach more because the interpreter can be built and composed with
other interpreters in completely separate modules or packages because they
are just values.

Rich

PS for the record, I don't strongly prefer the mtl style or the free monad
style, I think they each have good qualities and bad and which one I choose
tends to depend on other factors.

On Wed, Oct 19, 2016 at 10:26 AM, Christopher Allen <cma at bitemyapp.com>
wrote:

> It's not really more direct. It's an unordered collection of effects
> you can use. IME it's a less efficient mtl-style, but YMMV.
>
> Taking an example from a PureScript tutorial:
>
> func :: Eff (console :: CONSOLE, random :: RANDOM) Unit
>
> Can just as easily be:
>
> func :: (MonadConsole m, MonadGimmeRandom m) => m ()
>
> (mangled name so it doesn't overlap with a real class)
>
> There are other differences, but they haven't amounted to much for me yet.
>
> Kmett's Quine has a good example of some homespun mtl-style:
> https://github.com/ekmett/quine
>
> On Wed, Oct 19, 2016 at 12:17 PM, Will Yager <will.yager at gmail.com> wrote:
> > Can anyone comment on the use of Purescript-style effect monads as
> compared to MTL and Free? While I have not used them in practice, they seem
> to express the "intent" of monad composition a bit more directly than the
> approaches we use in Haskell.
> >
> > Cheers,
> > Will
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>
> --
> Chris Allen
> Currently working on http://haskellbook.com
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