test: add a new test absdiff3.go which uses function callback

We have disallowed having a typeparam on the right-hand-side of a type
declaration. So, we disabled much of the test absdiff.go. I recently
wrote a new test absdiff2.go to use a structure containing the type
param type, so I could attach a method properly and run the full test.

As a contrast, I thought I would create absdiff3.go, where the Abs
functionality is passed in as a function callback (but derived from a
generic function). This is simpler, and more inline with some of the
guidelines that Ian has been proposing (use passed-in functions rather
than requiring methods, when possible, for greater ease-of-use).

Only adds a new test absdiff3.go. (And fixes a comment in absdiff2.go.)

Change-Id: I6dd185b50a3baeec31f689a892319963468a7201
Reviewed-on: https://go-review.googlesource.com/c/go/+/380774
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>

test/typeparam/absdiff2.go

@@ -4,6 +4,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// absdiff example in which an Abs method is attached to a generic type, which is a
+// structure with a single field that may be a list of possible basic types.
+
 package main
 
 import (
@@ -24,7 +27,7 @@ type numericAbs[T Numeric] interface {
 	Abs() T
 }
 
-// AbsDifference computes the absolute value of the difference of
+// absDifference computes the absolute value of the difference of
 // a and b, where the absolute value is determined by the Abs method.
 func absDifference[T Numeric, U numericAbs[T]](a, b U) T {
 	d := a.Value - b.Value

test/typeparam/absdiff3.go

@@ -0,0 +1,82 @@
+// run -gcflags=-G=3
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// absdiff example using a function argument rather than attaching an
+// Abs method to a structure containing base types.
+
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type Numeric interface {
+	OrderedNumeric | Complex
+}
+
+// absDifference computes the absolute value of the difference of
+// a and b, where the absolute value is determined by the abs function.
+func absDifference[T Numeric](a, b T, abs func(a T) T) T {
+	return abs(a - b)
+}
+
+// OrderedNumeric matches numeric types that support the < operator.
+type OrderedNumeric interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
+		~float32 | ~float64
+}
+
+func Abs[T OrderedNumeric](a T) T {
+	if a < 0 {
+		return -a
+	}
+	return a
+}
+
+// Complex matches the two complex types, which do not have a < operator.
+type Complex interface {
+	~complex64 | ~complex128
+}
+
+func ComplexAbs[T Complex](a T) T {
+	r := float64(real(a))
+	i := float64(imag(a))
+	d := math.Sqrt(r*r + i*i)
+	return T(complex(d, 0))
+}
+
+// OrderedAbsDifference returns the absolute value of the difference
+// between a and b, where a and b are of an ordered type.
+func OrderedAbsDifference[T OrderedNumeric](a, b T) T {
+	return absDifference(a, b, Abs[T])
+}
+
+// ComplexAbsDifference returns the absolute value of the difference
+// between a and b, where a and b are of a complex type.
+func ComplexAbsDifference[T Complex](a, b T) T {
+	return absDifference(a, b, ComplexAbs[T])
+}
+
+func main() {
+	if got, want := OrderedAbsDifference(1.0, -2.0), 3.0; got != want {
+		panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	}
+	if got, want := OrderedAbsDifference(-1.0, 2.0), 3.0; got != want {
+		panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	}
+	if got, want := OrderedAbsDifference(-20, 15), 35; got != want {
+		panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	}
+
+	if got, want := ComplexAbsDifference(5.0+2.0i, 2.0-2.0i), 5+0i; got != want {
+		panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	}
+	if got, want := ComplexAbsDifference(2.0-2.0i, 5.0+2.0i), 5+0i; got != want {
+		panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	}
+}

test/typeparam/absdiffimp2.dir/a.go

@@ -21,7 +21,7 @@ type numericAbs[T Numeric] interface {
 	Abs() T
 }
 
-// AbsDifference computes the absolute value of the difference of
+// absDifference computes the absolute value of the difference of
 // a and b, where the absolute value is determined by the Abs method.
 func absDifference[T Numeric, U numericAbs[T]](a, b U) T {
 	d := a.Value - b.Value