docs for variants

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+! (c)2009 Joe Groff bsd license
+USING: arrays classes classes.singleton classes.tuple help.markup
+help.syntax kernel multiline slots quotations ;
+IN: variants
+
+HELP: VARIANT:
+{ $syntax <"
+VARIANT: class-name
+    singleton
+    singleton
+    tuple: { slot slot slot ... }
+    .
+    .
+    .
+    ; "> }
+{ $description "Defines " { $snippet "class-name" } " as a union of the following " { $link singleton-class } " and " { $link tuple-class } " definitions. Each " { $snippet "singleton" } " word is defined as a " { $snippet "singleton-class" } ", and each " { $snippet "tuple" } " word is defined aas a " { $snippet "tuple-class" } " with the given set of " { $snippet "slot" } "s, using the same syntax for slot specifiers as " { $link POSTPONE: TUPLE: } ". Typed tuple slots are able to recursively reference the variant " { $snippet "class-name" } " being defined. For " { $snippet "tuple" } " types, a " { $link boa } " constructor word " { $snippet "<tuple>" } " is defined as well." }
+{ $examples { $code <"
+USING: kernel variants ;
+IN: scratchpad
+
+VARIANT: list
+    nil
+    cons: { { first object } { rest list } }
+    ;
+"> } } ;
+
+HELP: match
+{ $values { "branches" array } }
+{ $description "Dispatches on the type of the value on the top of the stack. If the type is a " { $link singleton-class } ", the corresponding quotation is called with an empty stack. If the type is a " { $link tuple-class } ", the tuple slots are pushed onto the stack by order of arguments." }
+{ $examples { $example <"
+USING: kernel math prettyprint variants ;
+IN: scratchpad
+
+VARIANT: list
+    nil
+    cons: { { first object } { rest list } }
+    ;
+
+: list-length ( list -- length )
+    {
+        { nil [ 0 ] }
+        { cons [ nip list-length 1 + ] }
+    } match ;
+
+1 2 3 4 nil <cons> <cons> <cons> <cons> list-length .
+"> "4" } } ;
+
+HELP: unboa
+{ $values { "class" class } }
+{ $description "Decomposes a tuple of type " { $snippet "class" } " into its component slot values by order of arguments. The inverse of " { $link boa } "." } ;
+
+HELP: variant-class
+{ $class-description "This class comprises class names that have been defined with " { $link POSTPONE: VARIANT: } ". When a " { $snippet "variant-class" } " is used as the type of a specialized " { $link tuple } " slot, the variant's first member type is used as the default " { $link initial-value } "." } ;
+
+{ POSTPONE: VARIANT: variant-class match } related-words
+
+ARTICLE: "variants" "Algebraic data types"
+"The " { $vocab-link "variants" } " vocabulary provides syntax and combinators for defining and manipulating algebraic data types."
+{ $subsection POSTPONE: VARIANT: }
+{ $subsection variant-class }
+{ $subsection match } ;
+
+ABOUT: "variants"