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cl:special_operators:tagbody [2017/05/01 21:00] (current) |
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| + | ====== Special Operator TAGBODY ====== | ||
| + | |||
| + | ====Syntax==== | ||
| + | * **tagbody** //{tag | statement}''*''// → //*[[CL:Constant Variables:nil]]**// | ||
| + | |||
| + | ====Arguments and Values==== | ||
| + | * //tag// - a //[[CL:Glossary:go tag]]//; not evaluated. | ||
| + | * //statement// - a //[[CL:Glossary:compound form]]//; evaluatedspecial. | ||
| + | |||
| + | ====Description==== | ||
| + | Executes zero or more //[[CL:Glossary:statements]]// in a //[[CL:Glossary:lexical environment]]// that provides for control transfers to labels indicated by the //tags//. | ||
| + | |||
| + | The //statements// in a **tagbody** are //[[CL:Glossary:evaluate|evaluated]]// in order from left to right, and their //[[CL:Glossary:value|values]]// are discarded. If at any time there are no remaining //statements//, **tagbody** returns **[[CL:Constant Variables:nil]]**. | ||
| + | |||
| + | However, if ''(go //tag//)'' is //[[CL:Glossary:evaluate|evaluated]]//, control jumps to the part of the body labeled with the //tag//. (Tags are compared with **[[CL:Functions:eql]]**.) | ||
| + | |||
| + | A //tag// established by **tagbody** has //[[CL:Glossary:lexical scope]]// and has //[[CL:Glossary:dynamic extent]]//. Once **tagbody** has been exited, it is no longer valid to **[[CL:Special Operators:go]]** to a //tag// in its body. It is permissible for **[[CL:Special Operators:go]]** to jump to a **tagbody** that is not the innermost **tagbody** containing that **[[CL:Special Operators:go]]**; the //tags// established by a **tagbody** only shadow other //tags// of like name. | ||
| + | |||
| + | The determination of which elements of the body are //tags// and which are //statements// is made prior to any //[[CL:Glossary:macro expansion]]// of that element. If a //statement// is a //[[CL:Glossary:macro form]]// and its //[[CL:Glossary:macro expansion]]// is an //[[CL:Glossary:atom]]//, that //[[CL:Glossary:atom]]// is treated as a //statement//, not a //tag//. | ||
| + | |||
| + | ====Examples==== | ||
| + | <blockquote> | ||
| + | ([[CL:Special Operators:let]] ((val [[CL:Constant Variables:nil]])) | ||
| + | (tagbody | ||
| + | ([[CL:Macros:setf]] val 1) | ||
| + | ([[CL:Special Operators:go]] point-a) | ||
| + | ([[CL:Macros:incf]] val 16) | ||
| + | point-c | ||
| + | ([[CL:Macros:incf]] val 04) | ||
| + | ([[CL:Special Operators:go]] point-b) | ||
| + | ([[CL:Macros:incf]] val 32) | ||
| + | point-a | ||
| + | ([[CL:Macros:incf]] val 02) | ||
| + | ([[CL:Special Operators:go]] point-c) | ||
| + | ([[CL:Macros:incf]] val 64) | ||
| + | point-b | ||
| + | ([[CL:Macros:incf]] val 08)) | ||
| + | val) <r>15 </r> | ||
| + | | ||
| + | ([[CL:Macros:defun]] f1 (flag) | ||
| + | ([[CL:Special Operators:let]] ((n 1)) | ||
| + | (tagbody | ||
| + | ([[CL:Macros:setf]] n (f2 flag #'([[CL:Symbols:lambda]] () ([[CL:Special Operators:go]] out)))) | ||
| + | out | ||
| + | (prin1 n)))) <r>F1</r> | ||
| + | ([[CL:Macros:defun]] f2 (flag escape) | ||
| + | ([[CL:Special Operators:if ]]flag ([[CL:Functions:funcall]] escape) 2)) <r>F2 </r> | ||
| + | (f1 [[CL:Constant Variables:nil]]) | ||
| + | <o>2</o> | ||
| + | <r>[[CL:Constant Variables:nil|NIL]] </r> | ||
| + | (f1 [[CL:Constant Variables:t]]) | ||
| + | <o>1</o> | ||
| + | <r>[[CL:Constant Variables:nil|NIL]]</r> | ||
| + | </blockquote> | ||
| + | ====Affected By==== | ||
| + | None. | ||
| + | |||
| + | ====Exceptional Situations==== | ||
| + | None. | ||
| + | |||
| + | ====See Also==== | ||
| + | * **[[CL:Special Operators:go|Special Operator GO]]** | ||
| + | |||
| + | ====Notes==== | ||
| + | The //[[CL:Glossary:macro|macros]]// listed below have //[[CL:Glossary:implicit tagbody|implicit tagbodies]]//. | ||
| + | |||
| + | | [[CL:Macros:do]] | [[CL:Macros:do-external-symbols]] | [[CL:Macros:dotimes]] | | ||
| + | | [[CL:Macros:do*]] | [[CL:Macros:do-symbols]] | [[CL:Macros:prog]] | | ||
| + | | [[CL:Macros:do-all-symbols]] | [[CL:Macros:dolist]] | [[CL:Macros:prog*]] | | ||
| + | |||
| + | \issue{TAGBODY-TAG-EXPANSION:NO} | ||
