Using a Filter in a Shapeless, Scala

Question

It is easy to filter HList in formless type:

 val hlist = 1 :: 2 :: "3" :: true :: false :: HNil hlist.filter[Int] 

But how can I make my custom type filter? I want something like this: for example, I got a list of some functions:

 def function1(s: String) = s.toInt def function2(s: String) = s.toDouble def function3(i: Int) = i.toDouble val hflist = function1 _ :: function3 _ :: function2 _ :: HNil hflist customFilter[String] //> function1 _ :: function2 _ :: HNil 

So, after using this filter, a list of functions from the type String to some other type will be created.

I had an idea to use a card for this, but it was unsuccessful.

EDITION

Additional information about my comment:

I tried to test these ideas on a map:

So, if I have several lists (let's work with HList and hflist ):

 object allFunction extends Poly1 { implicit def default[T, M] = at[T => M](t => { object grabStringFunc extends skip { implicit def stringFunc[A] = at[T => A](_ :: HNil) } println(hflist flatMap grabStringFunc) //> here we should see result, list of functions }) hlist map allFunction //> result of this should be smth like (types) //> shapeless.::[Int => Double,shapeless.HNil]] //> shapeless.::[Int => Double,shapeless.HNil]] //> shapeless.::[String => Int,shapeless.::[String => Double,shapeless.HNil]] //> shapeless.HNil //> shapeless.HNil 

It is very interesting why it compiles and does not work correctly. In my opinion, this does not work, because the object cannot take prameters in this way ...

Answer 1

The easiest way is to use a fold. First we need a polymorphic function that will add each element to the battery if it is of the desired type ( String => A for some A ) and ignores it otherwise:

 trait ignore extends Poly2 { implicit def default[A, L <: HList] = at[A, L]((_, l) => l) } object keepStringFunc extends ignore { implicit def stringFunc[A, L <: HList] = at[String => A, L](_ :: _) } 

Now the following will give the result that you want in both versions 1.2.4 and 2.0.0-M1:

 val filtered = hflist.foldRight(HNil)(keepStringFunc) 

You can also write your own class type in the Filter , FilterAux (or Filter.Aux ) model, etc. - and it will be a good exercise if you are trying to find Shapeless-but foldRight much easier.

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Update: actually, what it costs for is a slightly more concise way to do this with flatMap :

 trait skip extends Poly1 { implicit def default[A] = at[A](_ => HNil) } object grabStringFunc extends skip { implicit def stringFunc[A] = at[String => A](_ :: HNil) } val filtered = hflist flatMap grabStringFunc 

I personally find the foldRight version more obvious, but this one is pretty elegant too.

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In response to your comment: you can make the solution more like the following:

 trait skip extends Poly1 { implicit def default[A] = at[A](_ => HNil) } trait grabFuncFrom[T] extends skip { implicit def stringFunc[A] = at[T => A](_ :: HNil) } object grabStringFunc extends grabFuncFrom[String] val filtered = hflist flatMap grabStringFunc 

But you still need the last step, in which you create a function of a higher rank as an object (see, for example, this answer , and Miles will comment on this to discuss this issue).