Merge pull request #2490 from SethTisue/dont-call-it-currying

Author: SethTisue

_tour/multiple-parameter-lists.md

@@ -1,6 +1,6 @@
 ---
 layout: tour
-title: Multiple Parameter Lists (Currying)
+title: Multiple Parameter Lists
 partof: scala-tour
 
 num: 12
@@ -96,3 +96,41 @@ println(squares) // List(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)
 val cubes = numberFunc((xs, x) => xs :+ x*x*x)
 println(cubes)  // List(1, 8, 27, 64, 125, 216, 343, 512, 729, 1000)
 ```
+
+### Comparison with "currying"
+
+You may sometimes see a method with multiple parameter lists referred to as "curried".
+
+As the [Wikipedia article on currying](https://en.wikipedia.org/wiki/Currying) states,
+
+> Currying is the technique of converting a function that takes
+> multiple arguments into a sequence of functions that each takes a
+> single argument
+
+We discourage the use of the word "curry" in reference to Scala's multiple parameter lists, for two reasons:
+
+1) In Scala, multiple parameters and multiple parameter lists are
+specified and implemented directly, as part of the language, rather
+being derived from single-parameter functions.
+
+2) There is danger of confusion with the Scala standard library's
+[`curried`](https://www.scala-lang.org/api/current/scala/Function2.html#curried:T1=%3E(T2=%3ER))
+and [`uncurried`](https://www.scala-lang.org/api/current/scala/Function$.html#uncurried[T1,T2,R](f:T1=%3E(T2=%3ER)):(T1,T2)=%3ER) methods, which don't involve multiple parameter lists at all.
+
+Regardless, there are certainly similarities to be found between
+multiple parameter lists and currying. Though they are different at
+the definition site, the call site might nonetheless look identical,
+as in this example:
+
+```scala mdoc
+// version with multiple parameter lists
+def addMultiple(n1: Int)(n2: Int) = n1 + n2
+// two different ways of arriving at a curried version instead
+def add(n1: Int, n2: Int) = n1 + n2
+val addCurried1 = (add _).curried
+val addCurried2 = (n1: Int) => (n2: Int) => n1 + n2
+// regardless, all three call sites are identical
+addMultiple(3)(4)  // 7
+addCurried1(3)(4)  // 7
+addCurried2(3)(4)  // 7
+```