diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 84 |
1 files changed, 46 insertions, 38 deletions
@@ -34500,7 +34500,7 @@ cells: .coNP Macro @ defset .synb -.mets (defset < name < params < new-val-sym < get-form << set-form ) +.mets (defset < name < params < new-val-sym << set-form ) .mets (defset < get-fun-sym << set-fun-sym ) .syne .desc @@ -34544,11 +34544,9 @@ only, which should ensure that the expression that holds is evaluated only once. The -.meta get-form -and .meta set-form -arguments specify forms which generate the code for, respectively, -accessing the value of the place, and storing a new value. +argument specifies an expression which generates the code for storing a new +value to the place. The .code defset @@ -34566,16 +34564,24 @@ syntax. Code is also generated to evaluate the expression which gives the new value to be stored, and that is bound to a generated variable whose name is bound to the .code new-val-sym -symbol. Then arrangements are made to invoke the -.code get-form -and +symbol. Then arrangements are made to invoke the operator named by +.meta name +and to evaluate the +.code set-form +in an environment in which these symbol bindings are visible. +The operator named +.meta name +is invoked using an altered argument list which uses temporary symbols in place +of the original expressions. The task of .code set-form -in an environment in which these symbol bindings are visible. The task of -these forms is to insert the values of the symbols from +is to insert the values of the symbols from .meta params and .meta new-val-sym -into suitable code templates that will perform the access and store actions. +into a suitable code templates that will perform the store actions. +The code generated by +.code set-form +must also take on the responsibility of yielding the new value as its result. If .meta params @@ -34631,8 +34637,6 @@ a temporary variable which holds the value of rather than to a list of temporary variable names holding the values of trailing expressions. The -.code get-form -and .code set-form must be prepared for this situation. In particular, the rest parameter's value is an atom, then it cannot be spliced in the backquote syntax, except at the @@ -34651,8 +34655,6 @@ the parameter expansion is applied at the time the .code defset form is processed, specifying an expanded parameter list which receives unevaluated expressions. The -.meta get-form -and .meta set-form may refer to all symbols produced by parameter list expansion, other than generated symbols. For instance, if a parameter list macro @@ -34668,8 +34670,6 @@ list of a then .code x will be visible to the -.meta get-form -and .metn set-form . The short, two-argument form of @@ -34733,21 +34733,20 @@ as a syntactic place using a long form .cblk (defset car (cell) new - ^(car ,cell) (let ((n (gensym))) ^(rlet ((,n ,new)) (progn (rplaca ,cell ,n) ,n)))) .cble Given such a definition, the expression -.code "(inc (car abc))" +.code "(inc (car (abc)))" expands to code closely resembling: .cblk - (let ((#:g0014 (abc))) - (let ((#:g0028 (succ (car #:g0014)))) - (rplaca #:g0014 #:g0028) - #:g0028)) + (let ((#:g0048 (abc))) + (let ((#:g0050 (succ (car #:g0048)))) + (rplaca #:g0048 #:g0050) + #:g0050)) .cble The @@ -34757,7 +34756,7 @@ macro has arranged for the argument expression of .code car to be evaluated to a temporary variable -.codn #:g0014 , +.codn #:g0048 , a .codn gensym . This, then, holds the @@ -34767,38 +34766,47 @@ At macro-expansion time, the variable .code cell from the parameter list specified by the .code defset -is bound to this symbol. The subexpression -.code "(car #:0014)" -is derived from the -.meta get-form -.code "^(car ,cell)" -which inserted the value of -.code cell -into a backquote template, that value being the symbol -.codn #:g0014 . +is bound to this symbol. The access expression +.code "(car #:0048)" +to retrieve the prior value is automatically generated +by combining the name of the place +.code car +with the gensym to which its argument +.code (abc) +has been evaluated. The .code new variable was bound to the expression giving the new value, namely -.codn "(succ (car #:g0014))" . +.codn "(succ (car #:g0048))" . The .meta set-form is careful to evaluate this only one time, storing its value into the temporary variable -.codn #:g0028 , +.codn #:g0050 , referenced by the variable .codn n . The .metn set-form 's .code "(rplaca ,cell ,n)" fragment thus turned into -.code "(rplaca #:g0014 #:g0028)" +.code "(rplaca #:g0048 #:g0050)" where -.code #:g0014 +.code #:g0048 references the cons cell being operated on, and -.code #:g0028 +.code #:g0050 the calculated new value to be stored into its .code car field. +The +.meta set-form +is careful to arrange for the new value +.code #:g0050 +to be returned. Those place-mutating operators which yield the new value, such +as +.code set +and +.code inc +rely on this behavior. .coNP Macro @ define-place-macro .synb |