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

Oh fair enough, I usually call the partially concreted or lensy style "mtl-style" and then call MonadReader/MonadState "mtl library". I lean to the former and usually use the latter if it discharges something I need right then and there. I've only used Free for something I needed to aggressively simulate/mock, and even then, some production uses of Free have gotten defenestrated in favor of something more ordinary. On Wed, Oct 19, 2016 at 3:47 PM, Richard Wallace wrote:

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 wrote:

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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 wrote:

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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 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Chris Allen Currently working on http://haskellbook.com _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Chris Allen Currently working on http://haskellbook.com