From 40ee866e90e7dc76bba86d73364dcfc9af2649a4 Mon Sep 17 00:00:00 2001
From: Miles Yucht <mgyucht@gmail.com>
Date: Sun, 26 Feb 2023 12:39:53 +0100
Subject: [PATCH] Correct example of expanded using clauses

The example of how using clauses are expanded by the compiler is not correct. `descending` accepts a context parameter for an ordering of the elements of the list, not an ordering of the list itself. I just tested this out in the repl:

```scala
scala> trait Ord[T]:
     |   def compare(x: T, y: T): Int
     |   extension (x: T)
     |     def < (y: T) = compare(x, y) < 0
     |     def > (y: T) = compare(x, y) > 0
     |
// defined trait Ord

scala> def max[T](x: T, y: T)(using ord: Ord[T]): T =
     |   if ord.compare(x, y) < 0 then y else x
     |
def max[T](x: T, y: T)(using ord: Ord[T]): T

scala> def maximum[T](xs: List[T])(using Ord[T]): T =
     |   xs.reduceLeft(max)
     |
def maximum[T](xs: List[T])(using x$2: Ord[T]): T

scala> given intOrd: Ord[Int] with
     |   def compare(x: Int, y: Int) =
     |     if x < y then -1 else if x > y then +1 else 0
     |
     | given listOrd[T](using ord: Ord[T]): Ord[List[T]] with
     |
     |   def compare(xs: List[T], ys: List[T]): Int = (xs, ys) match
     |     case (Nil, Nil) => 0
     |     case (Nil, _) => -1
     |     case (_, Nil) => +1
     |     case (x :: xs1, y :: ys1) =>
     |       val fst = ord.compare(x, y)
     |       if fst != 0 then fst else compare(xs1, ys1)
     |
// defined object intOrd
// defined class listOrd
def listOrd[T](using ord: Ord[T]): listOrd[T]

scala> val xs = List(1, 10, 2, 9, 3, 8, 4, 7, 5, 6)
val xs: List[Int] = List(1, 10, 2, 9, 3, 8, 4, 7, 5, 6)

scala> maximum(xs)
val res0: Int = 10

scala> def descending[T](using asc: Ord[T]): Ord[T] = new Ord[T]:
     |   def compare(x: T, y: T) = asc.compare(y, x)
     |
     | def minimum[T](xs: List[T])(using Ord[T]) =
     |   maximum(xs)(using descending)
     |
def descending[T](using asc: Ord[T]): Ord[T]
def minimum[T](xs: List[T])(using x$2: Ord[T]): T

scala> maximum(xs)(using descending)
val res1: Int = 1

scala> maximum(xs)(using descending(using intOrd))
val res2: Int = 1

scala> maximum(xs)(using descending(using listOrd))
-- [E007] Type Mismatch Error: -------------------------------------------------
1 |maximum(xs)(using descending(using listOrd))
  |                                   ^^^^^^^
  |                                   Found:    listOrd[Int]
  |                                   Required: Ord[Int]
  |
  | longer explanation available when compiling with `-explain`
1 error found
```
---
 docs/_docs/reference/contextual/using-clauses.md | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/docs/_docs/reference/contextual/using-clauses.md b/docs/_docs/reference/contextual/using-clauses.md
index 8f410ebc026e..f590cc2e7492 100644
--- a/docs/_docs/reference/contextual/using-clauses.md
+++ b/docs/_docs/reference/contextual/using-clauses.md
@@ -100,8 +100,7 @@ With this setup, the following calls are all well-formed, and they all normalize
 ```scala
 minimum(xs)
 maximum(xs)(using descending)
-maximum(xs)(using descending(using listOrd))
-maximum(xs)(using descending(using listOrd(using intOrd)))
+maximum(xs)(using descending(using intOrd))
 ```
 
 ## Multiple `using` Clauses