"By default, all slots are writable. If a slot is explicitly declared " { $link read-only } ", then no writer method is generated for the slot, and the only way to set it to a value other than its initial value is to construct an instance of the tuple with " { $link boa } ", passing the initial value for the read-only slot on the stack; the common idiom of calling " { $link new } " and then immediately filling in slot values with setter words will not work with read-only slots.";
ARTICLE: "slot-class-declaration""Slot class declarations"
"Class declaration is optional, and the default value is " { $link object } ", the class of all objects. If a more specific class is declared, then the object system maintains an invariant that the value of the slot must always be an instance of the class, even during construction. This invariant is enforced at a number of locations:"
{ $list
{ "Writer words (" { $link "accessors" } ") throw an error if the new value does not satisfy the class predicate." }
{ "The " { $link new } " word fills in slots with their initial values; the (per-class) initial values are required to satisfy the class predicate." }
{ "The " { $link boa } " word ensures that the values on the stack satisfy the class predicate." }
{ { $link "mirrors" } " ensure that the value passed to " { $link set-at } " satisfies the class predicate." }
{ "The " { $link slots>tuple } " and " { $link >tuple } " words ensure that the values in the sequence satisfy the correct class predicates." }
{ { $link "tuple-redefinition" } " fills in new slots with initial values and ensures that changes to existing declarations result in incompatible values being replaced with the initial value of their respective slots." }
"If the class of a slot is declared to be one of " { $link fixnum } " or " { $link float } ", then rather than testing values with the class predicate, writer words coerce values to the relevant type with " { $link >fixnum } " or " { $link >float } ". This may still result in error, but permits a wider range of values than a class predicate test. It also results in a possible loss of precision; for example, storing a large integer into a " { $link fixnum } " slot will silently overflow and discard high bits, and storing a ratio into a " { $link float } " slot may lose precision if the ratio is one which cannot be represented exactly with floating-point."
"This feature is mostly intended as an optimization for low-level code designed to avoid integer overflow, or where floating point precision is sufficient. Most code needs to work transparently with large integers, and thus should avoid the coercion behavior by using " { $link integer } " and " { $link real } " in place of " { $link fixnum } " and " { $link float } ".";
"A " { $emphasis "parametrized constructor" } " is a word which directly or indirectly calls " { $link new } " or " { $link boa } ", but instead of passing a literal class symbol, it takes the class symbol as an input from the stack."
"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>" } "."
"Constructors play a part in enforcing the invariant that slot values must always match slot declarations. The " { $link new } " word fills in the tuple with initial values, and " { $link boa } " ensures that the values on the stack match the corresponding slot declarations. See " { $link "tuple-declarations" } "."
"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 construct 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 new } ", " { $link boa } ", or a custom parametrized constructor."
"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."
"If the slot declaration of an existing slot changes, existing values are checked to see if they are still an instance of the required class. Any which are not are replaced by the initial value of that slot."
"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.";
"A " { $emphasis "protocol slot" } " is one which is assumed to exist by the implementation of a class, without being defined on the class itself. The burden is on subclasses (or mixin instances) to provide this slot."
$nl
"Protocol slots are defined using a parsing word:"
"Protocol slots are used where the implementation of a superclass needs to assume that each subclass defines certain slots, however the slots of each subclass are potentially declared with different class specializers, thus preventing the slots from being defined in the superclass."
"For example, the " { $link growable } " mixin provides an implementation of the sequence protocol which wraps an underlying sequence, resizing it as necessary when elements are added beyond the length of the sequence. It assumes that the concrete mixin instances define two slots, " { $snippet "length" } " and " { $snippet "underlying" } ". These slots are defined as protocol slots: " { $snippet "SLOT: length" } " and " { $snippet "SLOT: underlying" } ". "
"An alternate approach would be to define " { $link growable } " as a tuple class with these two slots, and have other classes subclass it as required. However, this rules out subclasses defining these slots with custom type declarations."
$nl
"For example, compare the definitions of the " { $link sbuf } " class,"
"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" } "?" } ". Initially, no specific words are defined for constructing new instances of the tuple. Constructors must be defined explicitly, and tuple slots are accessed via automatically-generated accessor words."
{ $description "Checks if two tuples have equal slot values. This is the default behavior of " { $link = } " on tuples, unless the tuple class subclasses " { $link identity-tuple } " or implements a method on " { $link equal? } ". In cases where equality has been redefined, this word can be used to get the default semantics if needed." } ;
{ $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 "\"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" }
{ $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" } " with all slots set to their initial values (see" { $link "tuple-declarations" } ")." }
{ $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." }
