20 lines
1.1 KiB
Factor
20 lines
1.1 KiB
Factor
! Copyright (C) 2017 Alexander Ilin.
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! See http://factorcode.org/license.txt for BSD license.
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USING: help.markup help.syntax kernel ;
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IN: classes.maybe
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ABOUT: "maybes"
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ARTICLE: "maybes" "Maybe classes"
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"A " { $snippet "maybe" } " is an anonymous union class (" { $link "unions" } ") of its members and the " { $link POSTPONE: f } " class. An object is an instance of a " { $snippet "maybe" } " class if it is either an instance of any of its participants, or " { $link POSTPONE: f } "." $nl
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"The " { $snippet "maybe" } " classes are used to declare typed slots that are optional for a tuple. Without this mechanism it would be an error to assign " { $link POSTPONE: f } " to such a typed slot, and therefore any tuple containing them without specifying an " { $link POSTPONE: initial: } " value could not be created with the word " { $link new } "." $nl
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"A slot with an empty " { $snippet "maybe{ }" } " class can only hold the " { $link POSTPONE: f } " value."
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{ $examples
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{ $example
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"TUPLE: test-maybe { value maybe{ fixnum string } } ;"
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"test-maybe new value>>"
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"f"
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}
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}
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{ $see-also "unions" POSTPONE: maybe{ } ;
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