"A " { $emphasis "parametrized constructor" } " is a word which directly or indirectly calls " { $link construct-empty } " or " { $link construct-boa } ", but instead of passing a literal class symbol, it takes the class symbol as an input from the stack."
$nl
"Parametrized constructors are useful in many situations, in particular with subclassing. For example, consider the following code:"
{ $code
"TUPLE: vehicle max-speed occupants ;"
""
": add-occupant ( person vehicle -- ) occupants>> push ;"
"The two constructors depend on the implementation of " { $snippet "vehicle" } " because they are responsible for initializing the " { $snippet "occupants" } " slot to an empty vector. If this slot is changed to contain a hashtable instead, there will be two places instead of one. A better approach is to use a parametrized constructor for vehicles:"
{ $code
"TUPLE: vehicle max-speed occupants ;"
""
": add-occupant ( person vehicle -- ) occupants>> push ;"
"By convention, construction logic is encapsulated in a word named after the tuple class surrounded in angle brackets; for example, the constructor word for a " { $snippet "point" } " class might be named " { $snippet "<point>" } "."
$nl
"All tuple construction should be done through constructor words, and construction primitives should be encapsulated and never called outside of the vocabulary where the class is defined, because this encourages looser coupling. For example, a constructor word could be changed to use memoization instead of always constructing a new instance, or it could be changed to construt a different class, without breaking callers."
"Rectangles, parallelograms and circles are all shapes. We support two operations on shapes:"
{ $list
"Computing the area"
"Computing the perimiter"
}
"Rectangles and parallelograms use the same algorithm for computing the area, whereas they use different algorithms for computing perimiter. Also, rectangles and parallelograms both have " { $snippet "width" } " and " { $snippet "height" } " slots. We can exploit this with subclassing:"
ARTICLE: "tuple-inheritance-anti-example""When not to use tuple subclassing"
"Tuple subclassing should only be used for " { $emphasis "is-a" } " relationships; for example, a car " { $emphasis "is a" } " vehicle, and a circle " { $emphasis "is a" } " shape."
{ $heading "Anti-pattern #1: subclassing for has-a" }
"Subclassing should not be used for " { $emphasis "has-a" } " relationships. For example, if a shape " { $emphasis "has a" } " color, then " { $snippet "shape" } " should not subclass " { $snippet "color" } ". Using tuple subclassing in inappropriate situations leads to code which is more brittle and less flexible than it should be."
$nl
"For example, suppose that " { $snippet "shape" } " inherits from " { $snippet "color" } ":"
{ $code
"TUPLE: color r g b ;"
"TUPLE: shape < color ... ;"
}
"Now, the implementation of " { $snippet "shape" } " depends on a specific representation of colors as RGB colors. If a new generic color protocol is devised which also allows HSB and YUV colors to be used, the shape class will not be able to take advantage of them without changes. A better approach is to store the color in a slot:"
{ $heading "Anti-pattern #2: subclassing for implementation sharing only" }
"Tuple subclassing purely for sharing implementations of methods is not a good idea either. If a class " { $snippet "A" } " is a subclass of a class " { $snippet "B" } ", then instances of " { $snippet "A" } " should be usable anywhere that an instance of " { $snippet "B" } " is. If this properly does not hold, then subclassing should not be used."
"The second is to use ad-hoc slot polymorphism. If two classes define a slot with the same name, then code which uses " { $link "accessors" } " can operate on instances of both objects, assuming the values stored in that slot implement a common protocol. This allows code to be shared without creating contrieved relationships between classes."
{ $heading "Anti-pattern #3: subclassing to override a method definition" }
"While method overriding is a very powerful tool, improper use can cause tight coupling of code and lead to difficulty in testing and refactoring. Subclassing should not be used as a means of ``monkey patching'' methods to fix bugs and add features. Only subclass from classes which were designed to be inherited from, and when writing classes of your own which are intended to be subclassed, clearly document that subclasses may and may not do. This includes construction policy; document whether subclasses should use " { $link construct-empty } ", " { $link construct-boa } ", or a custom parametrized constructor."
{ $see-also "parametrized-constructors" } ;
ARTICLE: "tuple-subclassing""Tuple subclassing"
"Tuple subclassing can be used to express natural relationships between classes at the language level. For example, every car " { $emphasis "is a" } " vehicle, so if the " { $snippet "car" } " class subclasses the " { $snippet "vehicle" } " class, it can " { $emphasis "inherit" } " the slots and methods of " { $snippet "vehicle" } "."
$nl
"To define one tuple class as a subclass of another, use the optional superclass parameter to " { $link POSTPONE:TUPLE: } ":"
"In addition to the slot reader and writer words which " { $link POSTPONE:TUPLE: } " defines for every tuple class, it is possible to construct and take apart entire tuples in a generic way."
"In the following, the " { $emphasis "direct slots" } " of a tuple class refers to the slot names specified in the " { $link POSTPONE:TUPLE: } " form defining the tuple class, and the " { $emphasis "effective slots" } " refers to the concatenation of the direct slots together with slots defined on superclasses."
$nl
"When a tuple class is redefined, all instances of the class, including subclasses, are updated. For each instance, the list of effective slots is compared with the previous list. If any slots were removed, the values are removed from the instance and are lost forever. If any slots were added, the instance gains these slots with an initial value of " { $link f } "."
$nl
"There are three ways to change the list of effective slots of a class:"
{ $list
"Adding or removing direct slots of the class"
"Adding or removing direct slots of a superclass of the class"
"Changing the inheritance hierarchy by redefining a class to have a different superclass"
}
"In all cases, the new effective slots are compared with the old effective slots, and each instance is updated as follows:"
{ $list
"If any slots were removed, the values are removed from the instance and are lost forever."
{ "If any slots were added, the instance gains these slots with an initial value of " { $link f } "." }
"If any slots are permuted, their values in instances do not change; only the layout of the instance changes in memory."
"If the number or order of effective slots changes, any BOA constructors are recompiled."
}
"Note that if a slot is moved from a class to its superclass (or vice versa) in the same compilation unit, the value of the slot is preserved in existing instances, because tuple instance update always runs at the end of a compilation unit. However, if it is removed in one compilation unit and added in another, the value in existing instances is lost.";
"Tuples are user-defined classes composed of named slots."
{ $subsection "tuple-examples" }
"A parsing word defines tuple classes:"
{ $subsection POSTPONE:TUPLE: }
"For each tuple class, several words are defined. First, there is the class word, a class predicate, and accessor words for each slot."
$nl
"The class word is used for defining methods on the tuple class; it has the same name as the tuple class. The predicate is named " { $snippet { $emphasis "name" } "?" } ". Tuple slots are accessed via accessor words:"
{ $warning "This word is in the " { $vocab-link "classes.tuple.private" } " vocabulary because it does not do any type checking. Passing values which are not tuples can result in memory corruption." } ;
{ $class-description "The class of tuples. This class is further partitioned into disjoint subclasses; each tuple shape defined by " { $link POSTPONE:TUPLE: } " is a new class."
$nl
"Tuple classes have additional word properties:"
{ $list
{ { $snippet "\"constructor\"" } " - a word for creating instances of this tuple class" }
{ { $snippet "\"predicate\"" } " - a quotation which tests if the top of the stack is an instance of this tuple class" }
{ { $snippet "\"slots\"" } " - a sequence of " { $link slot-spec } " instances" }
{ { $snippet "\"slot-names\"" } " - a sequence of strings naming the tuple's slots" }
{ { $snippet "\"tuple-size\"" } " - the number of slots" }
} } ;
HELP:define-tuple-predicate
{ $values { "class" tuple-class } }
{ $description "Defines a predicate word that tests if the top of the stack is an instance of " { $snippet "class" } ". This will only work if " { $snippet "class" } " is a tuple class." }
{ $description "If the new slot layout differs from the existing one, updates all existing instances of this tuple class, and forgets any slot accessor words which are no longer needed."
$nl
"If the class is not a tuple class word, this word does nothing." }
{ $values { "class" word } { "superclass" class } { "slots""a sequence of strings" } }
{ $description "Defines a tuple class inheriting from " { $snippet "superclass" } " with slots named by " { $snippet "slots" } ". This is the run time equivalent of " { $link POSTPONE:TUPLE: } "." }
{ $description "Creates a tuple with slot values taken from a sequence. The first element of the sequence must be a tuple class word and the remainder the declared slots."
{ $description "Outputs an array having the tuple's slots as elements. The first element is the tuple class word and remainder are declared slots." } ;
{ $description "Creates a new instance of " { $snippet "class" } ", storing consecutive stack values into the slots of the new tuple using setter words in " { $snippet "slots" } ". The top-most stack element is stored in the right-most slot." }
{ $description "Creates a new instance of " { $snippet "class" } " and fill in the slots from the stack, with the top-most stack element being stored in the right-most slot." }
{ $notes "The " { $snippet "-boa" } " suffix is shorthand for ``by order of arguments'', and ``BOA constructor'' is a pun on ``boa constrictor''." } ;
