Simplify expression

[Chris Marshall]

suppose I have 

type VS[A] = Validation[String, A]

val v: VS[Option[A]]

val f: A => VS[B]

I want to get a result of type VS[Option[B]] but if v is of type Success(none), the result should also be a success(None). Here's an example:

scala> val v: VS[Option[String]] = some("4.5").success

v: VS[Option[String]] = Success(Some(4.5))

scala> val f = (s : String) => (try { s.toInt.success } catch { case x => x.getMessage.fail }): VS[Int]

f: String => VS[Int] = <function1>

Then:

scala> import Validation.Monad._

import Validation.Monad._

scala> (v map2 f map (_.sequence)).join

res4: scalaz.Validation[String,Option[Int]] = Failure(For input string: "4.5")

The success case is:

scala> val v: VS[Option[String]]= some("5").success

v: VS[Option[String]] = Success(Some(5))

scala> (v map2 f map (_.sequence)).join

res7: scalaz.Validation[String,Option[Int]] = Success(Some(5))

And the empty case is:

scala> val v: VS[Option[String]]= none[String].success

v: VS[Option[String]] = Success(None)

scala> (v map2 f map (_.sequence)).join

res6: scalaz.Validation[String,Option[Int]] = Success(None)

Is there a "nicer" way of doing this (possibly involving kleisli composition or monad transformers)?

http://stackoverflow.com/questions/10853788/can-i-use-monad-transformers-to-simplify-this-composition

Chris

[Tony Morris]

Take a look at OptionT.

--
Tony Morris
http://tmorris.net/

[Chris Marshall]

is this:

OptionT(v) flatMap f

Sorry - I'm still on v6, so I'm not very familiar with it.

Chris

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[Lars Hupel]

`OptionT[F, T]` is basically equivalent to `F[Option[T]]`. If `F` is a
monad, then so is `OptionT[F, _]`.

However, it is not as easy as a simple `flatMap`, because for that, your
`f` has to have the type

String => OptionT[VS, Int]

Your `f` has the type

String => VS[Int]

Thus, you have to `lift` the result type of `f` into `OptionT` (or you
can rewrite `f`. `OptionT` has a `Pure` instance, so this should be easy).

[Lars Hupel]

> Thus, you have to `lift` the result type of `f` into `OptionT` (or you
> can rewrite `f`. `OptionT` has a `Pure` instance, so this should be easy).

Sorry, I meant to write "lift" instead of `lift`. I'm not sure whether
such a function already exists.

[eric thul]

hi Chris,

Using scalaz7, the following looks like it might do what you want:

scala> :paste

// Entering paste mode (ctrl-D to finish)

import scalaz._, Scalaz._

type VS[A] = Validation[String, A]

val f = (s : String) => (try { s.toInt.success } catch { case x => x.getMessage.fail }): VS[Int]

val u: VS[Option[String]] = some("4.5").success

val ru = OptionT(u).flatMapF((s:String) => f(s))(Validation.validationMonad).run

val v: VS[Option[String]]= some("5").success

val rv = OptionT(v).flatMapF((s:String) => f(s))(Validation.validationMonad).run

val w: VS[Option[String]]= none[String].success

val rw = OptionT(w).flatMapF((s:String) => f(s))(Validation.validationMonad).run

// Exiting paste mode, now interpreting.

import scalaz._

import Scalaz._

defined type alias VS

f: String => VS[Int] = <function1>

u: VS[Option[String]] = Success(Some(4.5))

ru: VS[Option[Int]] = Failure(For input string: "4.5")

v: VS[Option[String]] = Success(Some(5))

rv: VS[Option[Int]] = Success(Some(5))

w: VS[Option[String]] = Success(None)

rw: VS[Option[Int]] = Success(None)

Best,

 -Eric

[eric thul]

ps - Sorry, should be: OptionT(u).flatMapF(f)(Validation.validationMonad).run

[Chris Marshall]

Thanks Eric

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[eric thul]

welcome

Thanks Eric

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