How to slice a tuple in scala

Question

I am trying to slice a tuple, removing the last two items. I tried using the list drop/take methods but I can't succeed to get a tuple back. Here is the approach I tried : ``` scala> val myTuple = (1, 2, 4, 5, 0, 5) myTuple: (Int, Int, Int, Int, Int, Int) = (1,2,4,5,0,5) scala> val myList = myTuple.productIterator.toList myList: List[Any] = List(1, 2, 4, 5, 0, 5) scala> val mySubList = myList.dropRight(2) mySubList: List[Any] = List(1, 2, 4, 5) scala> val mySubTuple = ??? ``` I saw here that list to tuple isn't (yet?) possible in scala. Are there other ways to get that subtuple (without dealing with myTuple._1, myTuple._2...) ?

Accepted Answer

This is the sort of thing that shapeless can do in a generic way, involving conversion into an `HList`.

First - get shapeless. Then run scala with dependent method types switched on (on by default in 2.10):

```
C:\Scala\sdk\scala-2.9.2\bin>scala -Ydependent-method-types
Welcome to Scala version 2.9.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_04).
Type in expressions to have them evaluated.
Type :help for more information.
```

Add shapeless to the classpath:

```
scala> :cp C:\Users\cmarsha\Downloads\shapeless_2.9.2-1.2.2.jar
Added 'C:\Users\cmarsha\Downloads\shapeless_2.9.2-1.2.2.jar'.  Your new classpath is:
"C:	ibco	ibrv\8.2\lib	ibrvnative.jar;C:\Users\cmarsha\Downloads\shapeless_2.9.2-1.2.2.jar"
```

Now let us play!

```
scala> (1, 2.3, 'a, 'b', "c", true)
res0: (Int, Double, Symbol, Char, java.lang.String, Boolean) = (1,2.3,'a,b,c,true)
```

We must import shapeless

```
scala> import shapeless._; import Tuples._; import Nat._
import shapeless._
import Tuples._
import Nat._
```

We turn our tuple into an `HList`

```
scala> res0.hlisted
res2: shapeless.::[Int,shapeless.::[Double,shapeless.::[Symbol,shapeless.::[Char,shapeless.::[java.lang.String,shapeless.::[Boolean,shapeless.HNil]]]]]] = 1 :: 2.3 :: 'a :: b :: c :: true :: HNil
```

Then we take the first 4 (notice that `_4` is a type parameter, not a method argument)

```
scala> res2.take[_4]
res4: shapeless.::[Int,shapeless.::[Double,shapeless.::[Symbol,shapeless.::[Char, shapeless.HNil]]]] = 1 :: 2.3 :: 'a :: b :: HNil
```

Now convert back to a tuple

```
scala> res4.tupled
res5: (Int, Double, Symbol, Char) = (1,2.3,'a,b)
```

We could shorten this:

```
val (a, b, c, d) = sixtuple.hlisted.take[_4].tupled 
//a, b, c and d would all have the correct inferred type
```

This of course generalizes to the first `M` elements of an `N`-tuple

How to slice a tuple in scala

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