import scalaz._
import Scalaz._
"abc".parseInt
This will return a Validation[NumberFormatException, Int].
Is there a way I can apply a function on the failure side (such as toString) to get a Validation[String, Int]?
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2 Answers
19
There is a pair of methods <-: and :-> defined on MAB[M[_,_], A, B] that map on the left and right side of any M[A, B] as long as there is a Bifunctor[M]. Validation happens to be a bifunctor, so you can do this:
((_:NumberFormatException).toString) <-: "123".parseInt
Scala's type inference generally flows from left to right, so this is actually shorter:
"123".parseInt.<-:(_.toString)
And requires less annotation.
Apocalisp
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Nice. I wonder wether the following could be useful in MAB: def bimap[C,D](first: A => C = identity[A] \_, second: B => D = identity[B] \_)(implicit b: Bifunctor[M]): M[C, D] = b.bimap(value, first, second). So we get "123".parseInt.bimap(\_.toString) – Didier Dupont Sep 22 '11 at 23:17
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That would be awesome. Send a pull request. – Apocalisp Sep 23 '11 at 16:37
11
There is a functor on FailProjection. So you could do
v.fail.map(f).validation
(fail to type as FailProjection, validation to get out of it)
Alternatively
v.fold(f(_).failure, _.success)
Both a bit verbose. Maybe someone more familiar with scalaz can come up with something better
Apocalisp
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Didier Dupont
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can't get the `f andthen Failure` to work: `"123".parseInt.fold(_.toString andThen Failure)` prints `type mismatch`. The other one works fine. – huynhjl Sep 22 '11 at 16:11
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1@huynhjl: fold takes two functions: the first for the error case, the seconde for the success case. v.fold(_.toString.fail, _.success) works – Arjan Blokzijl Sep 22 '11 at 16:34
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But fold arguments have default values (identities) have they not ? – Didier Dupont Sep 22 '11 at 16:47
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@Apocalisp. Thanks.Now I have to find where failure is defined :-) – Didier Dupont Sep 22 '11 at 16:47
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foldArguments have default values: Ok, not the proper ones here, identity is not what we want – Didier Dupont Sep 22 '11 at 16:52
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_.failure and _.success are defined on Identity, like most other things that apply to types of kind `*`. – Apocalisp Sep 22 '11 at 16:57