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

It seems that there are several advantages to the Purescript approach. For example, this catchException :: forall a e . (Error -> Eff e a) -> Eff (err :: EXCEPTION | e) a -> Eff e a would be unwieldy to express using typeclasses, requiring at least three constraints. I also find this style easier to read than constraints, as it requires no mental substitution. E.g. if I see (Foo m, Bar m, Bar n) => Baz -> -> m a When I get to the end of the type, I have to go back to the beginning to figure out what m is. I can't read left-to-right. This happens a lot with constraint-based monad composition. Another advantage is that the Purescipt example uses a concrete type, which is often easier to reason about than "ad-hoc" typeclass abstractions like MonadRandom. However, it looks like you still get the flexibility of ad-hoc typeclasses, because you get to pick any function that discharges the effect type in the given effect monad. Like I said, I have not used it, but these are what I've noticed from topical observation. Apologies for the formatting; copying that code example appears to have confused the iOS mail app. Cheers, Will

On Oct 19, 2016, at 12:26, Christopher Allen 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 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 _______________________________________________ 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