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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 82 |
1 files changed, 82 insertions, 0 deletions
@@ -10812,6 +10812,88 @@ document, judiciously taking into account the content of the X3J13 Cleanup Issues named PRINT-CIRCLE-STRUCTURE:USER-FUNCTIONS-WORK and PRINT-CIRCLE-SHARED:RESPECT-PRINT-CIRCLE. +.NP* Notation for Erasing Objects + +.meIP #; < expr + +The \*(TL notation +.code #; +in TXR Lisp indicates that the expression +.meta expr +is to be read and then discarded, as if it were replaced by whitespace. + +This is useful for temporarily "commenting out" an expression. + +.TP* Notes: +Whereas it is valid for a \*(TL source file to be empty, it is +a syntax error if a \*(TL source file contains nothing but one or more +objects which are each suppressed by a preceding +.codn #; . +In the interactive listener, an input line consisting of nothing but +commented-out objects is similarly a syntax error. + +The notation does not cascade; consecutive occurrences of +.code #; +trigger a syntax error. + +The notation interacts with the circle notation. Firstly, if an object +which is erased by +.code #; +contains circular-referencing instances of the label notation, +those instances refer to +.codn nil . +Secondly, commented-out objects may introduce labels +which are subsequently referenced in +.metn expr . +An example of the first situation occurs in: + +.cblk + #;(#1=(#1#)) +.cble + +Here the +.code #1# +label is a circular reference because it refers to an object which +is a parent of the object which contains that reference. Such a reference +is only satisfied by a "backpatching" process once the entire surrounding syntax +is processed to the top level. The erasure perpetrated by +.code #; +causes the +.code #1# +label reference to be replaced by +.codn nil , +and therefore the labeled object is the object +.codn (nil) . + +An example of the second situation is + +.cblk + #;(#2=(a b c)) #2# +.cble + +Here, even though the expression +.code "(#2=(a b c))" +is suppressed, the label definition which it has introduced persists into the +following object, where the label reference +.code #2# +resolves to +.codn "(a b c)" . + +A combination of the two situations occurs in + +.cblk + #;(#1=(#1#)) #1# +.cble + +which yields +.codn "(nil)" . +This is because the +.code #1= +label is available; but the earlier +.code #1# +reference, being a circular reference inside an erased object, had lapsed to +.codn nil . + .SS* Generalization of List Accessors In ancient Lisp in the 1960's, it was not possible to apply the operations .code car |