[lectures] Update generics slides.

lectures/08-generics/lecture.slide

@@ -11,6 +11,29 @@ generics
 - Type sets
 - Type inference
 
+* Sorting in Go
+
+what we have
+
+  func Sort(data Interface)
+
+  type Interface interface {
+    Len() int
+    Less(i, j int) bool
+    Swap(i, j int)
+  }
+
+what we really want
+
+  func Sort(list []Elem)
+
+  // use
+  Sort(myList)
+
+* Type parameters to the rescue
+
+  func Sort[Elem ?](list []Elem)
+
 * Parameter lists
 
 An ordinary parameter list
@@ -23,6 +46,95 @@ A type parameter list
 
 - Convention: Type parameter names are capitalized
 
+* Constraints
+
+- A constraint specifies the requirements which a type argument must satisfy.
+- In generic Go, constraints are interfaces
+- A type argument is valid if it implements its constraint.
+
+* Generic Sort
+
+  func Sort[Elem interface{ Less(y Elem) bool }](list []Elem) {
+    ...
+  }
+
+- The constraint is an interface, but the actual type argument can be any type that implements that interface.
+- The scope of a type parameter starts at the opening "[" and ends at the end of the generic type or function declaration
+
+* Using generic Sort
+
+Somewhere in library
+
+  func Sort[Elem interface{ Less(y Elem) bool }](list []Elem)
+
+User code
+
+  type book struct{...}
+  func (x book) Less(y book) bool {...}
+
+  var bookshelf []book
+  ...
+  Sort[book](bookshelf) // generic function call
+
+* Type-checking the Sort call: Instantiation
+
+What happens when we call Sort?
+
+  Sort[book](bookshelf)
+
+- Substitution. Substitute book for elem
+
+  Sort[Elem interface{ Less(y Elem) bool }] | (list []Elem)
+
+  Sort[book interface{ Less(y book) bool }] | (list []book)
+
+- Verification. Verify that book satisfies the book parameter constraint
+
+- Instantiate book-specific function
+
+  #Sort[book] | (list []book)
+
+* Type-checking a generic call
+
+Instantiation (new)
+
+- replace type parameters with type arguments in entire signature
+- verify that each type argument satisfies its constraint
+
+Invocation (as usual)
+
+- verify that each ordinary argument can be assigned to its parameter
+
+* Types can be generic, too
+
+  type Lesser[T any] interface{
+    Less(y T) bool
+  }
+
+any stands for "no constraint" (same as "interface{}")
+
+* Sort, decomposed
+
+  type Lesser[T any] interface{
+    Less(y T) bool
+  }
+
+  func Sort[Elem Lesser[Elem]](list []Elem)
+
+* Problems
+
+what we want
+
+  Sort([]int{1, 2, 3})
+
+int does not implement Elem constraint (no Less method)
+
+what we could do
+
+  type myInt int
+
+  func (x myInt) Less(y myInt) bool { return x < y }
+
 * min
 
 .play -edit min/basic/min.go /^func min/,/^}/
@@ -125,185 +237,84 @@ A type parameter list
 - Type intefence is complicated but usage is simple
 - Programms that don't need type arguments today won't need them tomorrow
 
-* Sorting in Go
+* Scale
 
-what we have
+  type Point []uint32
 
-  func Sort(data Interface)
+  func (p Point) String() string { return "" }
 
-  type Interface interface {
-    Len() int
-    Less(i, j int) bool
-    Swap(i, j int)
+We'd like to write function scale and Println
+
+  func ScaleAndPrint(p Point) {
+  	r := Scale(p, 2)
+  	fmt.Println(r.String())
   }
 
-what we really want
+* Scale, first attempt
 
-  func Sort(list []Elem)
+.play -edit scale/wrong/scale.go /^func Scale/,/^}/
 
-  // use
-  Sort(myList)
+* Scale, first attempt
 
-* Type parameters to the rescue
+.play -edit scale/wrong/scale.go /^func Scale/,/^}/
 
-  func Sort[Elem ?](list []Elem)
-
-* Constraints
-
-- A constraint specifies the requirements which a type argument must satisfy.
-- In generic Go, constraints are interfaces
-- A type argument is valid if it implements its constraint.
-
-* Generic Sort
-
-  func Sort[Elem interface{ Less(y Elem) bool }](list []Elem) {
-    ...
+  func ScaleAndPrint(p Point) {
+  	r := Scale(p, 2)
+  	fmt.Println(r.String())
   }
 
-- The constraint is an interface, but the actual type argument can be any type that implements that interface.
-- The scope of a type parameter starts at the opening "[" and ends at the end of the generic type or function declaration
+- Compiler error: (type []uint32 has no field or method String)
 
-* Using generic Sort
+* Scale, fixed
 
-Somewhere in library
+.play -edit scale/fixed/scale.go /^func Scale/,/^}/
 
-  func Sort[Elem interface{ Less(y Elem) bool }](list []Elem)
+* Scale, fixed
 
-User code
+  func Scale[S ~[]E, E constraints.Integer](s S, c E) S
 
-  type book struct{...}
-  func (x book) Less(y book) bool {...}
+vs
 
-  var bookshelf []book
-  ...
-  Sort[book](bookshelf) // generic function call
+  func Scale[E constraints.Integer](s []E, c E) []E
 
-* Type-checking the Sort call: Instantiation
+* Inference
 
-  Sort[book] | (bookshelf)
-
-pass type argument
-
-  Sort[Elem interface{ Less(y Elem) bool }] | (list []Elem)
-
-substitute book for elem
-
-  Sort[book interface{ Less(y book) bool }] | (list []book)
-
-verify that book satisfies the book parameter constraint
-
-  #Sort[book] | (list []book)
-
-A generic function or type must be instantiated before it can be used.
-
-* Type-checking a generic call
-
-Instantiation (new)
-
-- replace type parameters with type arguments in entire signature
-- verify that each type argument satisfies its constraintThen, using the instantiated signature.
-
-Invocation (as usual)
-
-- verify that each ordinary argument can be assigned to its parameter
-
-* Types can be generic, too
-
-  type Lesser[T any] interface{
-    Less(y T) bool
+  func ScaleAndPrint(p Point) {
+  	r := Scale(p, 2)
+  	fmt.Println(r.String())
   }
 
-any stands for "no constraint" (same as "interface{}")
+Why don't we need explicit type parameters?
 
-* Sort, decomposed
+  	r := Scale[Point, int32](p, 2)
 
-  type Lesser[T any] interface{
-    Less(y T) bool
-  }
+* Inference
 
-  func Sort[Elem Lesser[Elem]](list []Elem)
-
-* Sort internals
-
-  func Sort[Elem interface{ Less(y Elem) bool }](list []Elem) {
-    ...
-    var i, j int
-    ...
-    if list[i].Less(List[j]) {...}
-    ...
-  }
-
-- type of list[i], list[j] is Elem
-- Elem is NOT an interface type!
-- A type parameter is a real type. It is not an interface type.
+- argument type inference
+- constraint type inference
 
 * Argument type inference
 
-what we have
+  func Scale[S ~[]E, E constraints.Integer](s S, c E) S
 
-  Sort[book](bookshelf)
+  type Point []int32
 
-what we want
+  Scale(p, 2)
 
-  Sort(bookshelf)
+- p is []Point => S is Point
+- 2 is untyped constant => no info
 
-Type unification
+* Constraint type inference
 
-  bookshelf -> []book
+  func Scale[S ~[]E, E constraints.Integer](s S, c E) S
 
-Inference
+  type Point []int32
 
-  func Sort[Elem ...]([]Elem) => Elem == book
+  Scale(p, 2)
 
-* Problems
-
-what we want
-
-  Sort([]int{1, 2, 3})
-
-int does not implement Elem constraint (no Less method)
-
-what we could do
-
-  type myInt int
-
-  func (x myInt) Less(y myInt) bool { return x < y }
-
-* Type lists
-
-A constraint interface may have a list of types (besides methods):
-
-  type Float interface {
-    type float32, float64
-  }
-
-  // Sin computes sin(x) for x of type float32 or float64.
-  func Sin[T Float](x T) T
-
-Satisfying a type list
-
-An argument type satisfies a constraint with a type list if
-
-- The argument type implements the methods of the constraint
-- The argument type or its underlying type is found in the type list.
-
-As usual, the satisfaction check happens after substitution.
-
-* Generic min function
-
-  type Ordered interface {
-    type int, int8, int16, ..., uint, uint8, uint16, ...,
-    float32, float64, string
-  }
-
-min internals
-
-  func min[T Ordered](x, y T) T {
-    if x < y {
-      return x
-    }
-    return y
-  }
+- S is Point (argument type inference)
+- S is defined in terms of E => we can infer E
+- E is int32
 
 * Different type parameters are different types
 

lectures/08-generics/min/basic/min.go

@@ -0,0 +1,8 @@
+package basic
+
+func min(x, y int) int {
+	if x < y {
+		return x
+	}
+	return y
+}

lectures/08-generics/min/generic/min.go

@@ -0,0 +1,10 @@
+package generic
+
+import "golang.org/x/exp/constraints"
+
+func min[T constraints.Ordered](x, y T) T {
+	if x < y {
+		return x
+	}
+	return y
+}

lectures/08-generics/scale/fixed/scale.go

@@ -0,0 +1,11 @@
+package wrong
+
+import "golang.org/x/exp/constraints"
+
+func Scale[S ~[]E, E constraints.Integer](s S, c E) S {
+	r := make(S, len(s))
+	for i, v := range s {
+		r[i] = v * c
+	}
+	return r
+}

lectures/08-generics/scale/wrong/scale.go

@@ -0,0 +1,11 @@
+package wrong
+
+import "golang.org/x/exp/constraints"
+
+func Scale[E constraints.Integer](s []E, c E) []E {
+	r := make([]E, len(s))
+	for i, v := range s {
+		r[i] = v * c
+	}
+	return r
+}