Constrained genericity

From Rosetta Code
Constrained genericity
You are encouraged to solve this task according to the task description, using any language you may know.

Constrained genericity or bounded quantification means that a parametrized type or function (see parametric polymorphism) can only be instantiated on types fulfilling some conditions, even if those conditions are not used in that function.

Say a type is called "eatable" if you can call the function eat on it. Write a generic type FoodBox which contains a collection of objects of a type given as parameter, but can only be instantiated on eatable types. The FoodBox shall not use the function eat in any way (i.e. without the explicit restriction, it could be instantiated on any type). The specification of a type being eatable should be as generic as possible in your language (i.e. the restrictions on the implementation of eatable types should be as minimal as possible). Also explain the restrictions, if any, on the implementation of eatable types, and show at least one example of an eatable type.


Ada allows various constraints to be specified in parameters of generics. A formal type constrained to be derived from certain base is one of them:

with Ada.Containers.Indefinite_Vectors;
package Nutrition is
type Food is interface;
procedure Eat (Object : in out Food) is abstract;
end Nutrition;
with Ada.Containers;
with Nutrition;
type New_Food is new Nutrition.Food;
package Food_Boxes is
package Food_Vectors is
new Ada.Containers.Indefinite_Vectors
( Index_Type => Positive,
Element_Type => New_Food
subtype Food_Box is Food_Vectors.Vector;
end Food_Boxes;

The package Nutrition defines an interface of an eatable object, that is, the procedure Eat. Then a generic container package is defined with the elements to be of some type derived from Food. Example of use:

type Banana is new Food with null record;
overriding procedure Eat (Object : in out Banana) is null;
package Banana_Box is new Food_Boxes (Banana);
type Tomato is new Food with null record;
overriding procedure Eat (Object : in out Tomato) is null;
package Tomato_Box is new Food_Boxes (Tomato);
-- We have declared Banana and Tomato as a Food.

The Tomato_Box can only contain tomatoes; the Banana_Box can only contain bananas. You can only create boxes of eatable objects.


Works with: C++11

Uses static assertion to disallow instantiations on incorrect types

template<typename T> //Detection helper struct
struct can_eat //Detects presence of non-const member function void eat()
template<typename U, void (U::*)()> struct SFINAE {};
template<typename U> static char Test(SFINAE<U, &U::eat>*);
template<typename U> static int Test(...);
static constexpr bool value = sizeof(Test<T>(0)) == sizeof(char);
struct potato
{ void eat(); };
struct brick
template<typename T>
class FoodBox
//Using static assertion to prohibit non-edible types
static_assert(can_eat<T>::value, "Only edible items are allowed in foodbox");
//Rest of class definition
int main()
FoodBox<potato> lunch;
//Following leads to compile-time error
//FoodBox<brick> practical_joke;


In C#, type constraints are made on the type hierarchy, so here we make IEatable an interface, with an Eat method. Types which are eatable would have to implement the IEatable interface and provide an Eat method.

interface IEatable
void Eat();

Type constraints in type parameters can be made via the where keyword, which allows us to qualify T. In this case, we indicate that the type argument must be a type that is a subtype of IEatable.

using System.Collections.Generic;
class FoodBox<T> where T : IEatable
List<T> food;

For example, an eatable Apple:

class Apple : IEatable
public void Eat()
System.Console.WriteLine("Apple has been eaten");

C# also has the interesting functionality of being able to require that a generic type have a default constructor. This means that the generic type can actually instantiate the objects without ever knowing the concrete type. To do so, we constrain the where clause with an additional term "new()". This must come after any other constraints. In this example, any type with a default constructor that implements IEatable is allowed.

using System.Collections.Generic
class FoodMakingBox<T> where T : IEatable, new()
List<T> food;
void Make(int numberOfFood)
{ = new List<T>();
for (int i = 0; i < numberOfFood; i++)
{ T());

Common Lisp

The technique used here is like that in the Abstract type task.

The task says that this task is only for statically typed languages, and Common Lisp is dynamically typed. However, there are many places where type declarations can be provided to the compiler, and there is user access to the type system (e.g., a user can ask whether an object is of a particular type). Via the latter mechanism, one could write a class containing a collection such that the insert method checked that the object to be inserted is of an appropriate type.

In this example, we define a class food, and two subclasses, inedible-food and edible-food. We define a generic function eat, and specialize it only for edible-food. We then define a predicate eatable-p which returns true only on objects for which eat methods have been defined. Then, using deftype with a satisfies type specifier, we define a type eatable to which only objects satisfying eatable-p belong. Finally, we define a function make-food-box which takes, in addition to typical array creation arguments, a type specifier. The array is declared to have elements of the type that is the intersection of food and the provided type. make-eatable-food-box simply calls make-food-box with the type eatable.

The only shortcoming here is that the compiler isn't required to enforce the type specifications for the arrays. A custom insert function, however, could remember the specified type for the collection, and assert that inserted elements are of that type.

(defclass food () ())
(defclass inedible-food (food) ())
(defclass edible-food (food) ())
(defgeneric eat (foodstuff)
(:documentation "Eat the foodstuff."))
(defmethod eat ((foodstuff edible-food))
"A specialized method for eating edible-food."
(format nil "Eating ~w." foodstuff))
(defun eatable-p (thing)
"Returns true if there are eat methods defined for thing."
(not (endp (compute-applicable-methods #'eat (list thing)))))
(deftype eatable ()
"Eatable objects are those satisfying eatable-p."
'(satisfies eatable-p))
(defun make-food-box (extra-type &rest array-args)
"Returns an array whose element-type is (and extra-type food).
array-args should be suitable for MAKE-ARRAY, and any provided
element-type keyword argument is ignored."

(destructuring-bind (dimensions &rest array-args) array-args
(apply 'make-array dimensions
:element-type `(and ,extra-type food)
(defun make-eatable-food-box (&rest array-args)
"Return an array whose elements are declared to be of type (and
eatable food)."

(apply 'make-food-box 'eatable array-args))


Template Version

enum IsEdible(T) = is(typeof(;
struct FoodBox(T) if (IsEdible!T) {
T[] food;
alias food this;
struct Carrot {
void eat() {}
static struct Car {}
void main() {
FoodBox!Carrot carrotsBox; // OK
carrotsBox ~= Carrot(); // Adds a carrot
//FoodBox!Car carsBox; // Not allowed

Interface Version

interface IEdible { void eat(); }
struct FoodBox(T : IEdible) {
T[] food;
alias food this;
class Carrot : IEdible {
void eat() {}
class Car {}
void main() {
FoodBox!Carrot carrotBox; // OK
//FoodBox!Car carBox; // Not allowed


It is surely arguable whether this constitutes an implementation of the above task:

/** Guard accepting only objects with an 'eat' method */
def Eatable {
to coerce(specimen, ejector) {
if (Ref.isNear(specimen) && specimen.__respondsTo("eat", 0)) {
return specimen
} else {
throw.eject(ejector, `inedible: $specimen`)
def makeFoodBox() {
return [].diverge(Eatable) # A guard-constrained list


Eiffel has included support for constrained genericty since its earliest implementations (as shown in Bertrand Meyer's paper from OOPSLA '86, available online.)

The "eatable" characteristic is modeled by a deferred class (deferred classes are similar to abstract classes in some other languages).

deferred class
feature -- Basic operations
-- Eat this eatable substance

Class EATABLE can then be inherited by any other class, with the understanding that the inheriting class will have to provide an implementation for the procedure eat. Here are two such classes, APPLE and PEAR:

feature -- Basic operations
-- Consume
print ("One apple eaten%N")

feature -- Basic operations
-- Consume
print ("One pear eaten%N")

Instances of the generic class FOOD_BOX can contain any types of EATABLE items. The constraint is shown in the formal generics part of the class declaration for FOOD_BOX:


So, any declaration of type FOOD_BOX can constrain its contents to any particular eatable type. For example:

my_apple_box: FOOD_BOX [APPLE]

The entity my_apple_box is declared as a FOOD_BOX which can contain only apples.

Of course, constraining a particular FOOD_BOX to all types which are eatable is also allowed, and could be appropriate in certain cases, such as:

my_refrigerator: FOOD_BOX [EATABLE]

Here's a small application that uses a FOOD_BOX constrained to contain only apples:

feature {NONE} -- Initialization
-- Run application.
create my_apple_box.make (10)
create one_apple
create one_pear
my_apple_box.extend (one_apple)
-- my_apple_box.extend (one_pear)
my_apple_box as ic
feature -- Access
my_apple_box: FOOD_BOX [APPLE]
-- My apple box
one_apple: APPLE
-- An apple
one_pear: PEAR
-- A pear

Notice that an instance of PEAR is also created, and a line of code is present as a comment which would attempt to place the pear in the apple box:

-- my_apple_box.extend (one_pear)

If the comment mark "--" were removed from this line of code, an compile error would occur because of the attempt to violate my_apple_bos's constraint.


It is possible to constrain type parameters in a number of ways, including inheritance relationships and interface implementation. But for this task, the natural choice is an explicit member constraint.

type ^a FoodBox                         // a generic type FoodBox
when ^a: (member eat: unit -> string) // with an explicit member constraint on ^a,
(items:^a list) = // a one-argument constructor
member inline x.foodItems = items // and a public read-only property
// a class type that fullfills the member constraint
type Banana() =
member = "I'm eating a banana."
// an instance of a Banana FoodBox
let someBananas = FoodBox [Banana(); Banana()]


Works with: Forth

Works with any ANS Forth

Needs the FMS-SI (single inheritance) library code located here:

include FMS-SI.f
include FMS-SILib.f
:class Eatable
 :m eat ." successful eat " ;m
\ FoodBox is defined without inspecting for the eat message
:class FoodBox
object-list eatable-types
 :m init: eatable-types init: ;m
 :m add: ( obj -- )
dup is-kindOf Eatable
if eatable-types add:
else drop ." not an eatable type "
then ;m
 :m test
begin eatable-types each:
while eat
repeat ;m
FoodBox aFoodBox
Eatable aEatable
aEatable aFoodBox add: \ add the e1 object to the object-list
aFoodBox test \ => successful eat
:class brick
 :m eat cr ." successful eat " ;m
brick abrick \ create an object that is not eatable
abrick aFoodBox add: \ => not an eatable type
:class apple <super Eatable
apple anapple
anapple aFoodBox add:
aFoodBox test \ => successful eat successful eat


In Fortran all checkes are done at compile time, in particular a dummy argument has to conform class.

module cg
implicit none
type, abstract :: eatable
end type eatable
type, extends(eatable) :: carrot_t
end type carrot_t
type :: brick_t; end type brick_t
type :: foodbox
class(eatable), allocatable :: food
procedure, public :: add_item => add_item_fb
end type foodbox
subroutine add_item_fb(this, f)
class(foodbox), intent(inout) :: this
class(eatable), intent(in) :: f
allocate(this%food, source=f)
end subroutine add_item_fb
end module cg
program con_gen
use cg
implicit none
type(carrot_t) :: carrot
type(brick_t) :: brick
type(foodbox) :: fbox
! Put a carrot into the foodbox
call fbox%add_item(carrot)
! Try to put a brick in - results in a compiler error
call fbox%add_item(brick)
end program con_gen

ifort -o cg cg.f90

cg.f90(40): error #6633: The type of the actual argument differs from the type of the dummy argument.   [BRICK]
    call fbox%add_item(brick)

gfortran -o cg cg.f90


    call fbox%add_item(brick)
Error: Type mismatch in argument 'f' at (1); passed TYPE(brick_t) to CLASS(eatable)


Go's interfaces do exactly what this task wants. Eatable looks like this:

type eatable interface {

And the following is all it takes to define foodbox as a slice of eatables. The result is that an object of type foodbox can hold objects of any type that implements the eat method (with the function signature specified in eatable.) The definition of foodbox though, doesn't even need to enumerate the functions of eatable, much less call them. Whatever is in the interface is okay.

type foodbox []eatable

Here is an example of an eatable type.

type peelfirst string
func (f peelfirst) eat() {
// peel code goes here
fmt.Println("mm, that", f, "was good!")

The only thing it takes to make peelfirst eatable is the definition of the eat method. When the eat method is defined, peelfirst automatically becomes an eatable. We say it satisfies the interface. Notice that "eatable" appears nowhere in the definition of peelfirst or the eat method of peelfirst.

Here is a complete program using these types.

package main
import "fmt"
type eatable interface {
type foodbox []eatable
type peelfirst string
func (f peelfirst) eat() {
// peel code goes here
fmt.Println("mm, that", f, "was good!")
func main() {
fb := foodbox{peelfirst("banana"), peelfirst("mango")}
f0 := fb[0]
mm, that banana was good!


A type class defines a set of operations that must be implemented by a type:

class Eatable a where
eat :: a -> String

We just require that instances of this type class implement a function eat which takes in the type and returns a string (I arbitrarily decided).

The FoodBox type could be implemented as follows:

data (Eatable a) => FoodBox a = F [a]

The stuff before the => specify what type classes the type variable a must belong to.

We can create an instance of Eatable at any time by providing an implementation for the function eat. Here we define a new type Banana, and make it an instance of Eatable.

data Banana = Foo -- the implementation doesn't really matter in this case
instance Eatable Banana where
eat _ = "I'm eating a banana"

We can declare existing types to be instances in the exact same way. The following makes Double an eatable type:

instance Eatable Double where
eat d = "I'm eating " ++ show d

Another way to make an existing type eatable is to declare all instances of another type class instances of this one. Let's assume we have another type class Food which looks like this;

class Food a where
munch :: a -> String

Then we can make all instances of Food eatable using munch for eat with the following instance declaration:

instance (Food a) => Eatable a where
eat x = munch x

Icon and Unicon

Neither Icon nor Unicon are statically typed. In Unicon, new types can be defined as classes. The solution shown follows the Scala approach.

import Utils        # From the UniLib package to get the Class class.
class Eatable:Class()
class Fish:Eatable(name)
method eat(); write("Eating "+name); end
class Rock:Class(name)
method eat(); write("Eating "+name); end
class FoodBox(A)
every item := !A do if "Eatable" == item.Type() then next else bad := "yes"
return /bad
procedure main()
if FoodBox([Fish("salmon")]) then write("Edible") else write("Inedible")
if FoodBox([Rock("granite")]) then write("Edible") else write("Inedible")

Sample run:




isEdible=:3 :0
create=:3 :0
collection=: 0#y
add=:3 :0"0
'inedible' assert isEdible_Connoisseur_ y
collection=: collection, y

An edible type would be a class that has a verb with the name 'eat' (the task "eatable" requirement is checked on an object or class reference using the static method isEdible_Connoisseur_).

We have also defined a 'FoodBox' container class which can only contain edible objects. (Our add method returns returns an empty result since its purpose is to add to the container, not to produce a result.)

For example:

eat=:3 :0

And here is a quicky demo of the above:

lunch=:'' conew 'FoodBox'
a1=: conew 'Apple'
a2=: conew 'Apple'
add__lunch a1
add__lunch a2
george=: conew 'Connoisseur'
add__lunch george
|inedible: assert


Works with: Java version 5

In Java type constraints are made on the type hierarchy, so here we make Eatable an interface, with an eat method. Types which are Eatable would have to implement the Eatable interface and provide an eat method.

interface Eatable
void eat();

Type constraints in type parameters can be made via the extends keyword, indicating in this case that the type argument must be a type that is a subtype of Eatable.

import java.util.List;
class FoodBox<T extends Eatable>
public List<T> food;

Similarly a generic method can constrain its type parameters

public <T extends Eatable> void foo(T x) { }
// although in this case this is no more useful than just "public void foo(Eatable x)"

This T does not necessarily have to be defined in the class declaration. Another method may be declared like this:

public class Test{
public <T extends Eatable> void bar(){ }

which has no indication of where T is coming from. This method could be called like this:


The foo method from before can figure out T from its parameter, but this bar method needs to be told what T is.


In the following program we define an interface, Eatable, and two classes - Cheese and Meat - which implement it and must therefore implement its eat() method because the interface itself does not provide a default implementation.

We then define a generic class, FoodBox, whose type parameter, T, is constrained to an Eatable type and instantiate it using both the Cheese and Meat types:

// version 1.0.6
interface Eatable {
fun eat()
class Cheese(val name: String) : Eatable {
override fun eat() {
println("Eating $name")
override fun toString() = name
class Meat(val name: String) : Eatable {
override fun eat() {
println("Eating $name")
override fun toString() = name
class FoodBox<T: Eatable> {
private val foodList = mutableListOf<T>()
fun add(food: T) {
override fun toString() = foodList.toString()
fun main(args: Array<String>) {
val cheddar = Cheese("cheddar")
val feta = Cheese("feta")
val cheeseBox = FoodBox<Cheese>()
println("CheeseBox contains : $cheeseBox")
val beef = Meat("beef")
val ham = Meat("ham")
val meatBox = FoodBox<Meat>()
println("MeatBox contains : $meatBox")
println("Full now!")
CheeseBox contains : [cheddar, feta]
MeatBox contains : [beef, ham]
Eating cheddar
Eating beef
Full now!


Translation of: D

Template Version

import morfa.type.traits;
template < T >
alias IsEdible = HasMember< T, "eat" >;
template < T >
if (IsEdible< T >)
struct FoodBox
var food: T[];
struct Carrot
func eat(): void {}
struct Car {}
func main(): void
var carrotBox: FoodBox< Carrot >; // OK ~= Carrot(); // Adds a carrot
// var carBox: FoodBox< Car >; // Not allowed
static assert( not trait(compiles, func() { var carBox: FoodBox< Car >; } ));

Interface Version

interface IEdible 
public func eat(): void;
template < T >
if (IsDerivedOf< T, IEdible >)
struct FoodBox
var food: T[];
class Carrot: IEdible
public override func eat(): void {}
class Car {}
func main(): void
var carrotBox: FoodBox< Carrot >; // OK
// var carBox: FoodBox< Car >; // Not allowed
static assert( not trait(compiles, func() { var carBox: FoodBox< Car >; } ));


using System.Collections.Generic;
interface IEatable
Eat() : void;
class FoodBox[T] : IEnumerable[T]
where T : IEatable
private _foods : list[T] = [];
public this() {}
public this(items : IEnumerable[T])
this._foods = $[food | food in items];
public Add(food : T) : FoodBox[T]
public GetEnumerator() : IEnumerator[T]
class Apple : IEatable
public this() {}
public Eat() : void
mutable appleBox = FoodBox();
repeat(3) {
appleBox = appleBox.Add(Apple());
foreach (apple in appleBox) apple.Eat();


Eatable = generic e
FoodBox[e: Eatable] = seq[e]
Food = object
name: string
count: int
proc eat(x: int) = echo "Eating the int: ", x
proc eat(x: Food) = echo "Eating ", x.count, " ",, "s"
var ints = FoodBox[int](@[1,2,3,4,5])
var fs = FoodBox[Food](@[])
fs.add Food(name: "Hamburger", count: 3)
fs.add Food(name: "Cheeseburger", count: 5)
for f in fs:


Works with: Xcode version 7

Type constraints are made on the type hierarchy, so here we make Eatable a protocol, with an eat method. Types which are Eatable would have to implement the Eatable protocol and provide an eat method.

@protocol Eatable
- (void)eat;

Type constraints in type parameters can be made via the : keyword, indicating in this case that the type argument must be a type that is a subtype of id<Eatable>.

@interface FoodBox<T : id<Eatable>> : NSObject


OCaml handles type constraints through modules and module types.

A module type defines a set of operations that must be implemented by a module:

module type Eatable = sig
type t
val eat : t -> unit

We just require that module instances of this module type describe a type t and implement a function eat which takes in the type and returns nothing.

The FoodBox generic type could be implemented as a functor (something which takes a module as an argument and returns another module):

module MakeFoodBox(A : Eatable) = struct
type elt = A.t
type t = F of elt list
let make_box_from_list xs = F xs

We can create a module that is an instance of Eatable by specifying a type providing an implementation for the function eat. Here we define a module Banana, and make it an instance of Eatable.

type banana = Foo (* a dummy type *)
module Banana : Eatable with type t = banana = struct
type t = banana
let eat _ = print_endline "I'm eating a banana"

We can also create modules that use an existing type as its t. The following module uses float as its type:

module EatFloat : Eatable with type t = float = struct
type t = float
let eat f = Printf.printf "I'm eating %f\n%!" f

Then, to make a FoodBox out of one of these modules, we need to call the functor on the module that specifies the type parameter:

module BananaBox = MakeFoodBox (Banana)
module FloatBox = MakeFoodBox (EatFloat)
let my_box = BananaBox.make_box_from_list [Foo]
let your_box = FloatBox.make_box_from_list [2.3; 4.5]

Unfortunately, it is kind of cumbersome in that, for every type parameter we want to use for this generic type, we will have to explicitly create a module for the resulting type (i.e. BananaBox, FloatBox). And the operations on that resulting type (i.e. make_box_from_list) are tied to each specific module.


ooRexx methods, routines, and collections are all untyped, so there are no language-level checks for type matches. Tests for identity need to be performed at runtime using mechanisms such as the object isA method.

call dinnerTime "yogurt"
call dinnerTime .pizza~new
call dinnerTime .broccoli~new
-- a mixin class that defines the interface for being "food", and
-- thus expected to implement an "eat" method
::class food mixinclass object
::method eat abstract
::class pizza subclass food
::method eat
Say "mmmmmmmm, pizza".
-- mixin classes can also be used for multiple inheritance
::class broccoli inherit food
::method eat
Say "ugh, do I have to?".
::routine dinnerTime
use arg dish
-- ooRexx arguments are typeless, so tests for constrained
-- types must be peformed at run time. The isA method will
-- check if an object is of the required type
if \dish~isA(.food) then do
say "I can't eat that!"
else dish~eat
I can't eat that!
mmmmmmmm, pizza.
ugh, do I have to?.


Generic but not too generic I trust.

macro Gluttony(vartype, capacity, foodlist)
typedef vartype physical
enum food foodlist
type ActualFood
sys name
physical size
physical quantity
end type
Class foodbox
has ActualFood Item[capacity]
sys max
method put(sys f, physical s,q)
end method
method GetNext(ActualFood *Stuff)
if max then
copy @stuff,@Item[max], sizeof Item
end if
end method
end class
Class Gourmand
physical WeightGain, SleepTime
method eats(ActualFood *stuff)
end method
end class
end macro
Gluttony (
% small 1
% medium 2
% large 3
% huge 7
% none 0
% single 1
% few 3
% several 7
% many 12
FoodBox Hamper
Gourmand MrG
Hamper.put food.pudding,large,several
Hamper.put food.pie,huge,few
ActualFood Course
Hamper.GetNext Course
MrG.eats Course
print MrG.WeightGain 'result 15.75

Perl 6

Works with: Rakudo version 2016.01
subset Eatable of Any where { .^can('eat') };
class Cake { method eat() {...} }
role FoodBox[Eatable] {
has %.foodbox;
class Yummy does FoodBox[Cake] { } # composes correctly
# class Yucky does FoodBox[Int] { } # fails to compose
my Yummy $foodbox .= new;
say $foodbox.perl;
Output: => {})


(class +Eatable)
(dm eat> ()
(prinl "I'm eatable") )
(class +FoodBox)
# obj
(dm set> (Obj)
(unless (method 'eat> Obj) # Check if the object is eatable
(quit "Object is not eatable" Obj) )
(=: obj Obj) ) # If so, set the object
(let (Box (new '(+FoodBox)) Eat (new '(+Eatable)) NoEat (new '(+Bla)))
(set> Box Eat) # Works
(set> Box NoEat) ) # Gives an error
$384320489 -- Object is not eatable

? (show Box)          
$384320487 (+FoodBox)
   obj $384320488

? (show Box 'obj)
$384320488 (+Eatable)

? (show NoEat)   
$384320489 (+Bla)


edible<%> objects simply need to state that they implement the interface in the second argument to class*. By doing so they will be forced to implement eat.

#lang racket
(module+ test (require tests/eli-tester))
;; This is all that an object should need to properly implement.
(define edible<%>
(interface () [eat (->m void?)]))
(define (generic-container<%> containee/c)
(interface ()
[contents (->m (listof containee/c))]
[insert (->m containee/c void?)]
[remove-at (->m exact-nonnegative-integer? containee/c)]
[count (->m exact-nonnegative-integer?)]))
(define ((generic-box-mixin containee/c) %)
(->i ([containee/c contract?])
(rv (containee/c) (implementation?/c (generic-container<%> containee/c))))
(class* % ((generic-container<%> containee/c))
(define l empty)
(define/public (contents) l)
(define/public (insert o) (set! l (cons o l)))
(define/public (remove-at i)
(begin0 (list-ref l i)
(append (take l i) (drop l (add1 i)))))
(define/public (count) (length l))))
;; As I understand it, a "Food Box" from the task is still a generic... i.e.
;; you will specify it down ;; to an "apple-box%" so: food-box%-generic is still
;; generic. food-box% will take any kind of food.
(define/contract (food-box-mixin T%)
(-> (or/c (λ (i) (eq? edible<%> i)) (implementation?/c edible<%>))
(generic-box-mixin (and/c (is-a?/c edible<%>) (is-a?/c T%))))
(module+ test
(define integer-box% ((generic-box-mixin integer?) object%))
(define integer-box (new integer-box%))
(define apple%
(class* object% (edible<%>)
(define/public (eat)
(displayln "nom!"))))
(define banana%
(class* object% (edible<%>)
(define/public (eat)
(displayln "peel.. peel... nom... nom!"))))
(define semolina%
(class* object% () ; <-- empty interfaces clause
 ;; you can call eat on it... but it's not explicitly (or even vaguely)
 ;; edible<%>
(define/public (eat) (displayln "blech!"))))
 ;; this will take any object that is edible<%> and edible<%> (therefore all
 ;; edible<%> objects)
(define any-food-box (new ((food-box-mixin edible<%>) object%)))
 ;; this will take any object that is edible and an apple<%>
 ;; (therefore only apple<%>s)
(define apple-food-box (new ((food-box-mixin apple%) object%)))
 ;; Test generic boxes
(send integer-box insert 22)
(send integer-box insert "a string") =error> exn:fail:contract?
 ;; Test the food box that takes any edible<%>
(send any-food-box insert (new apple%))
(send any-food-box insert (new banana%))
(send any-food-box insert (new semolina%)) =error> exn:fail:contract?
 ;; Test the food box that takes any apple%
(send apple-food-box insert (new apple%))
(send apple-food-box insert (new banana%)) =error> exn:fail:contract?
(send apple-food-box insert (new semolina%)) =error> exn:fail:contract?
(send apple-food-box count) => 1
 ;; Show that you cannot make a food-box from the non-edible<%> semolina cannot
(implementation? semolina% edible<%>) => #f
(new ((food-box-mixin semolina%) object%)) =error> exn:fail:contract?))

All the tests pass. Look at the tests to see what generates an exception (i.e. not allowed at runtime) and what does not.


class Foodbox
def initialize (*food)
raise ArgumentError, "food must be eadible" unless food.all?{|f| f.respond_to?(:eat)}
@box = food
class Fruit
def eat; end
class Apple < Fruit; end
# => #<Foodbox:0x00000001420c88 @box=[#<Fruit:0x00000001420cd8>, #<Apple:0x00000001420cb0>]>
p, "string can't eat")
# => test1.rb:3:in `initialize': food must be eadible (ArgumentError)


// This declares the "Eatable" constraint. It could contain no function.
trait Eatable {
fn eat();
// This declares the generic "FoodBox" type,
// whose parameter must satisfy the "Eatable" constraint.
// The objects of this type contain a vector of eatable objects.
struct FoodBox<T: Eatable> {
_data: Vec<T>,
// This implements the functions associated with the "FoodBox" type.
// This statement is not required, but here it is used
// to declare a handy "new" constructor.
impl<T: Eatable> FoodBox<T> {
fn new() -> FoodBox<T> {
FoodBox::<T> { _data: Vec::<T>::new() }
// This declares a simple type.
struct Banana {}
// This makes the "Banana" type satisfy the "Eatable" constraint.
// For that, every declaration inside the declaration of "Eatable"
// must be implemented here.
impl Eatable for Banana {
fn eat() {}
// This makes also the primitive "char" type satisfy the "Eatable" constraint.
impl Eatable for char {
fn eat() {}
fn main() {
// This instantiate a "FoodBox" parameterized by the "Banana" type.
// It is allowed as "Banana" implements "Eatable".
let _fb1 = FoodBox::<Banana>::new();
// This instantiate a "FoodBox" parameterized by the "char" type.
// It is allowed, as "char" implements "Eatable".
let _fb2 = FoodBox::<char>::new();
// This instantiate a "FoodBox" parameterized by the "bool" type.
// It is NOT allowed, as "bool" does not implement "Eatable".
//let _fb3 = FoodBox::<bool>::new();


abstract class $EDIBLE is
class FOOD < $EDIBLE is
readonly attr name:STR;
eat is
#OUT + "eating " + + "\n";
create(name:STR):SAME is
res ::= new; := name;
return res;
class CAR is
readonly attr name:STR;
create(name:STR):SAME is
res ::= new; := name;
return res;
class FOODBOX{T < $EDIBLE} is
private attr list:LLIST{T};
create:SAME is
res ::= new;
res.list := #;
return res;
add(c :T) is
elt!:T is loop yield self.list.elt!; end; end;
class MAIN is
main is
box  ::= #FOODBOX{FOOD}; -- ok
box.add(#FOOD("Amanita Muscaria"));
box2 ::= #FOODBOX{CAR}; -- not ok
box2.add(#CAR("Punto")); -- but compiler let it pass!
-- eat everything
loop box.elt!.eat; end;

The GNU Sather compiler v1.2.3 let the "box2" pass, even though it shouldn't. Read e.g. this tutorial's section


Scala can constrain types in many different ways. This specific constrain, for the type to contain a particular method, can be written as this:

type Eatable = { def eat: Unit }
class FoodBox(coll: List[Eatable])
case class Fish(name: String) {
def eat {
println("Eating "+name)
val foodBox = new FoodBox(List(new Fish("salmon")))

A more extensive discussion on genericity in Scala and some of the constrains that can be imposed can be found on Parametric Polymorphism.


class FoodBox(*food { .all { .respond_to(:eat) } }) { }
class Fruit { method eat { ... } }
class Apple < Fruit { }
say FoodBox(Fruit(), Apple()).dump #=> FoodBox(food: [Fruit(), Apple()])
say FoodBox(Apple(), "foo") #!> ERROR: class `FoodBox` !~ (Apple, String)


Here we make Eatable a protocol, with an eat method. Types which are Eatable would have to conform to the Eatable protocol and provide an eat method.

protocol Eatable {
func eat()

Type constraints in type parameters can be made via the : syntax, indicating in this case that the type argument must be a type that is a subtype of Eatable.

struct FoodBox<T: Eatable> {
var food: [T]

Similarly a generic function or method can constrain its type parameters

func foo<T: Eatable>(x: T) { }
// although in this case this is no more useful than just "func foo(x: Eatable)"


zkl isn't statically typed so the test is done at runtime.

This is a slightly different take on the task, keeping the editables and rejecting the garbage.

class Eatable{ var v;
fcn eat{ println("munching ",; }
class FoodBox{
fcn init(food1,food2,etc){
var contents=editable;
if(garbage) println("Rejecting: ",garbage);
class Apple(Eatable){} class Nuts(Eatable){} class Foo{}
Rejecting: L("boogers",Class(Foo))
munching Apple
munching Nuts