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cl:functions:set-dispatch-macro-character [2019/09/14 05:00]
cl:functions:set-dispatch-macro-character [2020/02/27 18:00] (current)
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 +  * **get-dispatch-macro-character** //​disp-char//​ //​sub-char//​ ''&​optional''​ //​readtable//​ → //​function//​
 +  * **set-dispatch-macro-character** //​disp-char//​ //​sub-char//​ //​new-function//​ ''&​optional''​ //​readtable//​ → **[[CL:​Constant Variables:​t]]**
 +====Arguments and Values====
 +  * //​disp-char//​ - a //​[[CL:​Glossary:​character]]//​.
 +  * //​sub-char//​ - a //​[[CL:​Glossary:​character]]//​.
 +  * //​readtable//​ - a //​[[CL:​Glossary:​readtable designator]]//​. The default is the //​[[CL:​Glossary:​current readtable]]//​.
 +  * //​function//​ - a //​[[CL:​Glossary:​function designator]]//​ or **[[CL:​Constant Variables:​nil]]**.
 +  * //​new-function//​ - a //​[[CL:​Glossary:​function designator]]//​.
 +**set-dispatch-macro-character** causes //​new-function//​ to be called when //​disp-char//​ followed by //​sub-char//​ is read.
 +If //​sub-char//​ is a lowercase letter, it is converted to its uppercase equivalent. It is an error if //​sub-char//​ is one of the ten decimal digits.
 +**set-dispatch-macro-character** installs a //​new-function//​ to be called when a particular //​[[CL:​Glossary:​dispatching macro character]]//​ pair is read. //​New-function//​ is installed as the dispatch function to be called when //​readtable//​ is in use and when //​disp-char//​ is followed by //​sub-char//​.
 +For more information about how the //​new-function//​ is invoked, see section {\secref\MacroChars}.
 +**get-dispatch-macro-character** retrieves the dispatch function associated with //​disp-char//​ and //​sub-char//​ in //​readtable//​.
 +**get-dispatch-macro-character** returns the macro-character function for //​sub-char//​ under //​disp-char//,​ or **[[CL:​Constant Variables:​nil]]** if there is no function associated with //​sub-char//​. If //​sub-char//​ is a decimal digit, **get-dispatch-macro-character** returns **[[CL:​Constant Variables:​nil]]**.
 +<​blockquote> ​
 +(get-dispatch-macro-character #\# #\{) <​r>​[[CL:​Constant Variables:​NIL]] </r>
 +(set-dispatch-macro-character #\# #\{ ;dispatch on #{ 
 +  #'​([[CL:​Symbols:​lambda]] (s c n) 
 +      ([[CL:​Special Operators:​let]] ((list ([[CL:​Functions:​read]] s [[CL:​Constant Variables:​nil]] ([[CL:​Functions:​values]]) [[CL:​Constant Variables:​t]]))) ;list is object after #n{ 
 +        ([[CL:​Macros:​when]] ([[CL:​Functions:​consp]] list) ;return nth element of list 
 +          ([[CL:​Macros:​unless]] ([[CL:​Macros:​and]] n ([[CL:​Functions:​math-less|<​]] 0 n ([[CL:​Functions:​length]] list))) ​
 +            ([[CL:​Macros:​setf]] n 0))
 +          ([[CL:​Macros:​setf]] list ([[CL:​Functions:​nth]] n list)))
 +        list))) <​r>​[[CL:​Constant Variables:​T]] </r>
 +#{(1 2 3 4) <r>1 </r>
 +#3{(0 1 2 3) <r>3 </r>
 +#{123 <​r>​123</​r>​
 +If it is desired that ''#​$//​foo//''​ was read as if it were ''​(dollars //​foo//​)''​.
 +([[CL:​Macros:​defun]] |#$-reader| (stream subchar arg) 
 +  ([[CL:​Symbols:​declare]] ([[CL:​Declarations:​ignore]] subchar arg)) 
 +  ([[CL:​Functions:​list]] '​dollars ([[CL:​Functions:​read]] stream [[CL:​Constant Variables:​t]] [[CL:​Constant Variables:​nil]] [[CL:​Constant Variables:​t]]))) <​r>​|#''​-reader| </r>
 +(set-dispatch-macro-character #\# #\$ #'​|#​$-reader|) <​r>​[[CL:​Constant Variables:​T]] </r>
 +====Affected By====
 +====Side Effects====
 +The //​readtable//​ is modified.
 +====Exceptional Situations====
 +For either function, an error is signaled if //​disp-char//​ is not a //​[[CL:​Glossary:​dispatching macro character]]//​ in //​readtable//​.
 +====See Also====
 +  * **[[CL:​Variables:​star-readtable-star|Variable *READTABLE*]]**
 +  * {\secref\MacroChars}
 +It is necessary to use **[[CL:​Functions:​make-dispatch-macro-character]]** to set up the dispatch character before specifying its sub-characters.