71 lines
3.8 KiB
Factor
71 lines
3.8 KiB
Factor
USING: help.markup help.syntax kernel classes classes.private words
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checksums checksums.crc32 sequences math ;
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IN: classes.algebra
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ARTICLE: "class-operations" "Class operations"
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"Set-theoretic operations on classes:"
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{ $subsections
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class=
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class<
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class<=
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class-and
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class-or
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classes-intersect?
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flatten-class
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} ;
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ARTICLE: "class-linearization" "Class linearization"
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"Classes have an intrinsic partial order; given two classes A and B, we either have that A is a subset of B, B is a subset of A, A and B are equal as sets, or they are incomparable. The last two situations present difficulties for method dispatch:"
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{ $list
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"If a generic word defines a method on a mixin class A and another class B, and B is the only instance of A, there is an ambiguity because A and B are equal as sets; any object that is an instance of one is an instance of both."
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{ "If a generic word defines methods on two union classes which are incomparable but not disjoint, for example " { $link sequence } " and " { $link number } ", there is an ambiguity because the generic word may be called on an object that is an instance of both unions." }
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}
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"The first ambiguity is resolved with a tie-breaker that compares metaclasses. The intrinsic meta-class order, from most-specific to least-specific:"
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{ $list
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"Built-in classes and tuple classes"
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"Predicate classes"
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"Union classes"
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"Mixin classes"
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}
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"This means that in the above example, the generic word with methods on a mixin and its sole instance will always call the method for the sole instance, since it is more specific than a mixin class."
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$nl
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"The second problem is resolved with another tie-breaker. When performing the topological sort of classes, if there are multiple candidates at any given step of the sort, lexicographical order on the class name is used."
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$nl
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"Operations:"
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{ $subsections
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class<
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sort-classes
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smallest-class
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}
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"Metaclass order:"
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{ $subsections rank-class } ;
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HELP: flatten-class
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{ $values { "class" class } { "assoc" "an assoc whose keys are classes" } }
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{ $description "Outputs a set of builtin and tuple classes whose union is the smallest cover of " { $snippet "class" } "." } ;
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HELP: class<=
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{ $values { "first" "a class" } { "second" "a class" } { "?" "a boolean" } }
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{ $description "Tests if all instances of " { $snippet "class1" } " are also instances of " { $snippet "class2" } "." }
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{ $notes "Classes are partially ordered. This means that if " { $snippet "class1 <= class2" } " and " { $snippet "class2 <= class1" } ", then " { $snippet "class1 = class2" } ". Also, if " { $snippet "class1 <= class2" } " and " { $snippet "class2 <= class3" } ", then " { $snippet "class1 <= class3" } "." } ;
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HELP: sort-classes
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{ $values { "seq" "a sequence of class" } { "newseq" "a new seqence of classes" } }
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{ $description "Outputs a linear sort of a sequence of classes. Larger classes come before their subclasses." } ;
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HELP: class-or
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{ $values { "first" class } { "second" class } { "class" class } }
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{ $description "Outputs the smallest anonymous class containing both " { $snippet "class1" } " and " { $snippet "class2" } "." } ;
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HELP: class-and
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{ $values { "first" class } { "second" class } { "class" class } }
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{ $description "Outputs the largest anonymous class contained in both " { $snippet "class1" } " and " { $snippet "class2" } "." } ;
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HELP: classes-intersect?
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{ $values { "first" class } { "second" class } { "?" "a boolean" } }
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{ $description "Tests if two classes have a non-empty intersection. If the intersection is empty, no object can be an instance of both classes at once." } ;
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HELP: smallest-class
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{ $values { "classes" "a sequence of class words" } { "class/f" { $maybe class } } }
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{ $description "Outputs a minimum class from the given sequence." } ;
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