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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 127 |
1 files changed, 127 insertions, 0 deletions
@@ -12755,6 +12755,21 @@ Note that .meta string is always required, and is always the rightmost argument. +.meIP >> [ struct << arg *] +The structure instance +.meta struct +is inquired whether it supports a method named by the symbol +.metn lambda . +If so, that method is invoked on the object. The method +receives +.meta struct +followed by the value of every +.metn arg . +If this form is used as a place, then the object must +support a +.code lambda-set +method. + .RE .PP @@ -22469,6 +22484,118 @@ is evaluated only once: (mapcar s list) <--> (mapcar (meth s lambda) list) .cble +Note: a form such as +.code "[s args ...]" +where +.code s +is a structure can be treated as a place if the method +.code lambda-set +is also implemented. + +.coNP Method @ lambda-set +.synb +.mets << object .(lambda-set << arg * << new-value) +.syne +.desc +The +.code lambda-set +method, in conjunction with a +.code lambda +method, allows structures to be used as place accessors. If +structure +.code s +supports a +.meta lambda-set +with four arguments, then the following use of the +.code dwim +operator is possible: + +.cblk + (set [s a b c d] v) + (set (dwim s a b c d) v) ;; precisely equivalently +.cble + +This has an effect which can be described by the following code: + +.cblk + (progn + (set s s.(lambda-set a b c d v)) + v) +.cble + +except that +.code s +and +.code v +are evaluated only once, and +.code a +through +.code d +are evaluated using the Lisp-1 semantics due the +.code dwim +operator. + +If a place-mutating operator is used on this form which requires the prior +value, such as the +.code inc +macro, then the structure must support the +.code lambda +function also. + +If +.code lambda +takes +.I n +arguments, then +.code lambda-set +should take +.I n+1 +arguments. The first +.I n +arguments of these two methods are congruent; the extra rightmost argument +of +.code lambda-set +is the new value to be stored into the place denoted by the prior +arguments. + +The return value of +.code lambda-set +is significant. Unless there is a very good reason for the method to +do otherwise, it should return the structure itself. This is because +the place-mutating operators store this returned value back to the place +which holds the structure itself. + +.TP* Example + +The following defines a structure with a single instance +slot +.code hash +which holds a hash table, as well as +.code lambda +and +.code lambda-set +methods: + +.cblk + (defstruct hash-wrapper nil + (hash (hash)) + + (:method lambda (self key) + [self.hash key]) + + (:method lambda-set (self key new-val) + (set [self.hash key] new-val) self)) +.cble + +An instance of this structure can now be used as follows: + +.cblk + (let ((s (new hash-wrapper))) + (set [s "apple"] 3 + [s "orange] 4) + [s "apple"]) -> 3 +.cble + .coNP Methods @, car @, cdr and @ nullify .synb .mets << object .(car) |