Comments on: Lifting

By: λ Tony’s blog λ » Blog Archive » Lifting (Haskell addendum)

λ Tony’s blog λ » Blog Archive » Lifting (Haskell addendum) — Fri, 19 Aug 2011 01:03:42 +0000

[...] follow-on from Lifting. A port of the Scala code to Haskell follows. class Lift f where lift0 :: a -> f a lift1 [...]

By: Tony Morris

Tony Morris — Mon, 25 Jul 2011 21:34:48 +0000

Germán, I’d use the next best thing available. Pattern-matching is a last resort, but in the absence of specialised combinators I might just have to resort that low in abstraction.

By: Germán

Germán — Mon, 25 Jul 2011 15:01:15 +0000

I tried ListLift and OptionLift, for which I tend to use for comprehensions. But these in turn are based on flatMap/map, right? If there was no flatMap/map, I guess you’d go for imperative iteration and pattern matching. Am I off the mark?

By: Tony Morris

Tony Morris — Wed, 20 Jul 2011 21:26:04 +0000

Hi Kenbot, yes a monad gives rise to all of the operations, however, a monad is not necessary. You may weak the interface to an applicative functor, which has some desirable properties that monads do not.

Channing, you’d just reimplement the bodies of flatMap/map in your definition of ap. If you’re comfortable with knowing how it all works, just use flatMap/map.

By: Channing Walton

Channing Walton — Wed, 20 Jul 2011 08:51:45 +0000

Hi Tony,
you mentioned that using map/flatmap is cheating. Any chance of a hint how to implement ap without flatmap/map?

Thanks

Channing

By: Kenbot

Kenbot — Tue, 19 Jul 2011 12:50:55 +0000

So it looks like most of these can be replaced with a class MonadLift[M[_]: scalaz.Monad] extends LiftImpl[M].

I’m not sure if this can be applied to the Function1 example though. Is it possible?

By: Paulo

Paulo — Mon, 18 Jul 2011 21:40:11 +0000

Thank you! I did try to annotate with Either[Char, Int] before, but it wasn’t working. My fault though, I had 2.7 installed instead of 2.9, and it seems I can drop the type-annotation entirely.

By: Tony Morris

Tony Morris — Mon, 18 Jul 2011 07:12:13 +0000

Hey Paulo, You’ll have to type-annotate your Left(’a') value.

By: Erik

Erik — Sun, 17 Jul 2011 13:35:36 +0000

Thanks Tony! This is short and very sweet, just the way I like it. Really enjoyed this one too, btw: http://blog.tmorris.net/understanding-practical-api-design-static-typing-and-functional-programming.

By: Paulo

Paulo — Sun, 17 Jul 2011 11:16:14 +0000

I’m having some trouble with the third one. Possibly I’m not understanding the type correctly, but shouldn’t the following typecheck:

val el = new EitherLift[Char]
val f1 = (x: Int) => x + 2
el.lift1(f1)(Right(13)) // Ok, gives the expected result Right(15)
el.lift1(f1)(Left(’a')) // Fails to typecheck