CODEWITHSUNDEEP
scala
user3248346
user3248346

Reputation:

How to create lift function

I am trying to create the following lift function but I am hitting a brick wall:

scala> def liftOff[A,B](f : A => B) : A => Option[B] = _.map(f)
<console>:10: error: value map is not a member of type parameter A
       def liftOff[A,B](f : A => B) : A => Option[B] = _.map(f)

I thought I could denote that the type parameter A is mappable but the mappable trait is now deprecated since 2.10. If the input type of the returned function is an Option it's not a problem but that is not what I need:

scala> def liftOff[A,B](f : A => B) : Option[A]  => Option[B] = _ map f
liftOff: [A, B](f: A => B)Option[A] => Option[B]

Is there a way to do this?

Upvotes: 2

Views: 104

Answers (2)

Andy Hayden
Andy Hayden

Reputation: 375885

Another option is to lift* it to the Option type first:

scala> def liftOff[A,B](f : A => B) : A => Option[B] = Some(_) map f
liftOff: [A, B](f: A => B)A => Option[B]

scala> liftOff((x:Int) => 2.0 * x)(3)
res6: Option[Double] = Some(6.0)

*Note: Here Some is the unit/return method lifting A to a monadic (Option) type.

Upvotes: 0

Shadowlands
Shadowlands

Reputation: 15074

If what you are trying to define is a method that takes a function from A to B, and returns a function from A to Option[B], then you just need to combine f with a function to enclose the result in an Option, such as:

def liftOff[A,B](f : A => B) : A => Option[B] = f andThen Option.apply

if this isn't what you meant, then you might need to edit your question to make your intent clearer.

Upvotes: 3

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