diff options
| -rw-r--r-- | txr.1 | 40 |
1 files changed, 36 insertions, 4 deletions
@@ -26939,8 +26939,15 @@ is used for the corresponding element of the range. When a .code sub form is used as a syntactic place, that place denotes a slice of -.metn seq , -which must be a syntactic place itself. +.metn seq . +The +.meta seq +argument must be itself be syntactic place, because +receives a new value, which may be different from its original value in +cases when +.meta seq +is a list. + Overwriting that slice is equivalent to using the .code replace function. The following equivalences give the semantics, except that @@ -26952,13 +26959,38 @@ and are evaluated only once, in left-to-right order: .verb - (set (sub x a b) v) <--> (progn (replace x v a b) + (set (sub x a b) v) <--> (progn (set x (replace x v a b)) v) (del (sub x a b)) <--> (prog1 (sub x a b) - (replace x nil a b)) + (set x (replace x nil a b))) .brev +Note that the value of +.code x +is overwritten with the value returned by +.codn replace . +If +.code x +is a vector or string, then the return value of +.code replace +is just +.codn x : +the identity of the object doesn't change under mutation. +However, if +.code x +is a list, its identity changes when items are added to or removed from +the front of the list, and in those cases +.code replace +will return a value different from its first argument. +Similarly, if +.code x +is an object with a +.code lambda-set +method, that method's return value becomes the return value of +.code replace +and must be taken into account. + .coNP Function @ replace .synb .mets (replace < sequence < replacement-sequence >> [ from <> [ to ]]) |