comma

Type:	-	reader expansion
Source:	-	xlcont.c, xlread.c

Syntax

(comma expr)

expr - an expression which is evaluated within a backquoted expression

Description

A backquote special form returns an expression unevaluated, except that portions of the expression may be evaluated when they are preceeded by a 'comma' [,] or comma-at [,@]. 'comma' will evaluate the portion of the expression the comma preceeds. If the portion is an atom or a list, it is placed as is within the expression.

Examples

(setq box 'stuff-inside)           ; BOX contains STUFF-INSIDE
(print box)                        ; prints STUFF-INSIDE
(quote (i have the box))           ; returns (I HAVE THE BOX)
(backquote (i have the box))       ; returns (I HAVE THE BOX)
(backquote (I have (comma box)))   ; returns (I HAVE STUFF-INSIDE)
(backquote (I have the ,@box))     ; returns (I HAVE THE)
(setq automobile '(a van))         ; set up AUTOMOBILE
(backquote (I have automobile))    ; returns (I HAVE AUTOMOBILE)
(backquote (I have ,automobile))   ; returns (I HAVE (A VAN))
(backquote (I have ,@automobile))  ; returns (I HAVE A VAN)
`(I have ,@automobile)             ; returns (I HAVE A VAN)

Read macro: XLISP supports the normal read macro of a comma [,] as a short-hand method of writing the 'comma' read-expansion.

Note: backquote , 'comma' and comma-at are very useful in defining macros via defmacro.

See the comma reader expansion in the XLISP 2.0 manual.

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