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| author | Kaz Kylheku <kaz@kylheku.com> | 2011-12-02 15:39:43 -0800 |
|---|---|---|
| committer | Kaz Kylheku <kaz@kylheku.com> | 2011-12-02 15:39:43 -0800 |
| commit | 63211dfd66f3cbd4159885018257022b20a4c975 (patch) | |
| tree | 3f38e75739cd8e2f0cdb9e33c903968805580e95 /txr.1 | |
| parent | 6768cdf7a46856355ccfafc57f147fac14eba06b (diff) | |
| download | txr-63211dfd66f3cbd4159885018257022b20a4c975.tar.gz txr-63211dfd66f3cbd4159885018257022b20a4c975.tar.bz2 txr-63211dfd66f3cbd4159885018257022b20a4c975.zip | |
* parser.y (list): unquote and splice actions look inside the
argument form. If an unquote or splice are applied to a quoted
form, its quote becomes a regular quote.
This behavior is necessary to make ,',form work in nested
quotes, otherwise the ' is a quasiquote which captures
the comma in ,form, reducing ,',form to ,form.
* txr.1: Documented this special behavior.
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 26 |
1 files changed, 26 insertions, 0 deletions
@@ -4225,6 +4225,10 @@ of the variable a. Note that TXR Lisp does not have a distinct quote and backquote syntax. There is only one quote, which supports unquoting. +A quoted form which contains no unquotes codifies an ordinary quote. + +A quoted form which contains unquotes expresses a quasiquote. + .IP ,form Thes comma character is used within a quoted list to denote an unquote. Wheras @@ -4241,6 +4245,28 @@ the form which follows ,* must evaluate to a list. That list is spliced into the quoted list. For example: '(a b c ,*(list (+ 3 3) (+ 4 4) d) evaluates to (a b c 6 8 d). The expression (list (+ 3 3) (+ 4 4)) is evaluated to produce the list (6 8), and this list is spliced into the quoted template. + +.IP ,'form + +The comma-quote combination has a special meaning: the quote always +behaves as a regular quote and not a quasiquote, even if form contains +unquotes. Therefore, it does not "capture" these unquotes: they cannot +"belong" to this quote. The comma and quote "cancel out", so the only effect +of comma-quote is to add one level of unquoting. So for instance, whereas in +'(a b c '(,d)), the subsitution of d belongs to the inner quote (it is unquoted +by the leftmost comma which belongs to the innermost quote) by contrast, +in '(a b c '(,',d)) the d is now one comma removed from the leftmost comma and +thus the substitution of d belongs to the outer quote. +In other dialects of Lisp, this would be written `(a b c `(,',d)), making it +explicit which kind of quote is being specified. TXR Lisp works out which +kind of quote to use internally. + +.IP ,*'form + +The comma-splice form is analogous to comma-quote (see above). Like in the +,' combination, in the ,*' combination, the quote behaves as a regular quote +and not a quasiquote. + .PP .SS Nested Quotes |