Exercise 2.1.7.1 honour comment

[andreas-roehler]

@winitzki commented [...]

In chapter02/worksheets/solution2.1.7.1_AR.scala:

@@ -0,0 +1,17 @@
[...]
+Exercise 2.1.7.1

• Find all integer pairs 𝑖, 𝑗 where 0 ≤ 𝑖 ≤ 9 and 0 ≤ 𝑗 ≤ 9 and 𝑖 + 4 ∗ 𝑗 > 𝑖 ∗ 𝑗.
• Hint: use flatMap and filter.
• */

+val a = (0 to 9).flatMap(x => (0 to 9).map { y => (x, y) } )
+val b = a.map{ case x => x._1 + 4 * x._2 }

I would suggest to use case (x, y) => here as well.

[andreas-roehler]

BTW will do that as exercise, however, not convinced WRT rationale behind. Because using pattern match of case seems more generic, that way losing information of the current type in question, which specific tuple-access provides.

[winitzki]

The reason is that { case (x, y) => ...} will fail to match (and fail to compile) unless the type is actually a tuple. { case x => ...} will never fail to match.

Suppose you are writing x.map { case x => x._1 + 4 * x._2 } where you expect x to have type Seq[(Int, Int)]. But, by mistake, x has type Seq[(Int, Int, Int)]. Your code with case x will still match and compile but {case (x, y) => ...} will fail to compile and will tell you that the types don't match.

So, writing {case (x, y) => is actually more specific and less generic.

[andreas-roehler]

Okay, did a little check with List:

scala> val x = List(9, 3)
x: List[Int] = List(9, 3)

scala> val y = List(5, 6)
y: List[Int] = List(5, 6)

scala> val z = List(x) ++ List(y)
z: List[List[Int]] = List(List(9, 3), List(5, 6))

scala> z.map{ case (i, j) => }
<console>:13: error: constructor cannot be instantiated to expected type;
 found   : (T1, T2)
 required: List[Int]
       z.map{ case (i, j) => }

scala> z.map{ case i => i }
res13: List[List[Int]] = List(List(9, 3), List(5, 6))

scala> z.map{ case List(i, j) => i }
res14: List[Int] = List(9, 5)