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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 34 |
1 files changed, 34 insertions, 0 deletions
@@ -5003,6 +5003,40 @@ In TXR Lisp, the / character can occur in symbol names, and the / token is a symbol. Therefore the /regex/ syntax is absent, replaced with the #/regex/ syntax. +.SS Generalization of List Accessors car and cdr + +In ancient Lisp in the 1960's, it was not possible to apply the operations +car and cdr to the nil symbol (empty list), because it is not a cons cell. In +the InterLisp dialect, this restriction was lifted: these operations were +extended to accept nil (and return nil). The convention was adopted in +other Lisp dialects and in Common Lisp. Thus there exists an object which +is not a cons, yet which takes car and cdr. + +In TXR Lisp, this concept is extended further. For the sake of convenience, +the operations car and cdr, are extended to work with strings and vectors: + + (cdr "") -> nil + (car "") -> nil + + (car "abc") -> #\ea + (cdr "abc") -> "bc" + + (cdr #(1 2 3)) -> #(2 3) + (car #(1 2 3)) -> 1 + +The ldiff function is also extended in a special way. When the right parameter +is a string or vector, then it uses the equal equality test rather than eq +for detecting the tail of the list. + + (ldiff "abcd" "cd") -> (#\ea #\eb) + +The ldiff operation starts with "abcd" and repeatedly applies cdr to produce +"bcd" and "cd", until the suffix is equal to the second argument: (equal "cd" +"cd") yields true. + +Operations based on car, cdr and ldiff, such as keep-if and remq extend to +strings and vectors. + .SH CONTROL FLOW AND SEQUENCING When the first element of a compound expression is an operator symbol, |