5 ms·

Why? Paul Graham says that "Programming languages teach us not to want what they cannot provide". I'm just curious if this is the case with lisp and c[a|d]+r

mahmud 17y ago
No it's not. If there is anything Lisp provides out of the box, it's support for non-builtin language features. A friken paradox, enjoy it! :-)
- philh 17y ago
  So far there's only one solution: "this can be done (for common lisp) by handling undefined-function condition and invoking appropriate restart (which is unfortunately implementation-specific)." Which makes it sound like there is in fact no way to do this in standard CL, to say nothing of other lisps. Untested perl code to do this (taking a cons to be a [car, cdr] array reference): AUTOLOAD { unless ($AUTOLOAD =~ m/::c([ad]{1,})r$/) { die "Attempted to call undefined function $AUTOLOAD"; } my $ads = $1; my $cons = shift; while (my $c = chop $ads) { $cons = $cons->[$c eq 'a' ? 0 : 1]; } return $cons; } Example of a situation this feature might be useful: "I have this object with lots of properties which I don't necessarily know what they are. How do I define getter/setter methods on them?"
  - bvttf 17y ago
    More appropriately, there's no standard way to do this with CL. One might ask the same about threads. What are threads? We just don't know.
    - mahmud 17y ago
      What do you mean there is no standard way to do this? There is a MetaObject Protocol which is supported by most platforms, and for those that don't have it, there is the widely portable CloserMOP. Threads are not "standard" but the last thread-less Common Lisp just got them this month. Every maintained Common Lisp implementation today has Threads; and all support Bordeaux-Threads. Show me a single mainstream language where threads are "standardized" other than Java?
      - bvttf 17y ago
        You're over-thinking this. My comment was a response to the idea that every lisp can that but there's no single standard. My awkward phrasing was a reference to about 1:53 in this: http://www.youtube.com/watch?v=bCWA7uevo_Q http://www.youtube.com/watch?v=bCWA7uevo_Q
  - mahmud 17y ago
    So far there's only one solution: "this can be done (for common lisp) by handling undefined-function condition and invoking appropriate restart (which is unfortunately implementation-specific)." Before you go far and wide on speculation alone, you might stop and reread what has been posted so far, again. Both me and lnostdal submitted working CL solutions. Example of a situation this feature might be useful: "I have this object with lots of properties which I don't necessarily know what they are. How do I define getter/setter methods on them?" ;; if you know the slot names (slot-value object 'slot-name) ==> value (setf (slot-value object 'slot-name) new-value) ;; if you don't. ;; let's define a class hierarchy first CL-USER> (defclass foo () ((a) (b))) #<STANDARD-CLASS FOO> CL-USER> (defclass bar (foo) ((c) (d))) #<STANDARD-CLASS BAR> CL-USER> (defclass baz (bar) ((e) (f))) #<STANDARD-CLASS BAZ> ;; use COMPUTE-SLOTS to get the class's slots CL-USER> (closer-mop:compute-slots (find-class 'baz)) (#<STANDARD-EFFECTIVE-SLOT-DEFINITION A #x1A0F62C5> #<STANDARD-EFFECTIVE-SLOT-DEFINITION B #x1A0F6305> #<STANDARD-EFFECTIVE-SLOT-DEFINITION C #x1A0F6345> #<STANDARD-EFFECTIVE-SLOT-DEFINITION D #x1A0F6385> #<STANDARD-EFFECTIVE-SLOT-DEFINITION E #x1A0F63C5> #<STANDARD-EFFECTIVE-SLOT-DEFINITION F #x1A0F6405>) ;; if you have an object and not a class, do the same CL-USER> (closer-mop:compute-slots (class-of (make-instance 'baz))) (#<STANDARD-EFFECTIVE-SLOT-DEFINITION A #x1A1202A1> #<STANDARD-EFFECTIVE-SLOT-DEFINITION B #x1A1202E1> #<STANDARD-EFFECTIVE-SLOT-DEFINITION C #x1A120321> #<STANDARD-EFFECTIVE-SLOT-DEFINITION D #x1A120361> #<STANDARD-EFFECTIVE-SLOT-DEFINITION E #x1A1203A1> #<STANDARD-EFFECTIVE-SLOT-DEFINITION F #x1A1203E1>)
rsheridan6 17y ago
I don't think so. c[a|d]+r is already too confusing with the builtin functions when there are more than a few [a|d]'s. There are better ways to get at items in a list.

mahmud 17y ago

No it's not. If there is anything Lisp provides out of the box, it's support for non-builtin language features. A friken paradox, enjoy it! :-)

philh 17y ago
So far there's only one solution: "this can be done (for common lisp) by handling undefined-function condition and invoking appropriate restart (which is unfortunately implementation-specific)." Which makes it sound like there is in fact no way to do this in standard CL, to say nothing of other lisps. Untested perl code to do this (taking a cons to be a [car, cdr] array reference): AUTOLOAD { unless ($AUTOLOAD =~ m/::c([ad]{1,})r$/) { die "Attempted to call undefined function $AUTOLOAD"; } my $ads = $1; my $cons = shift; while (my $c = chop $ads) { $cons = $cons->[$c eq 'a' ? 0 : 1]; } return $cons; } Example of a situation this feature might be useful: "I have this object with lots of properties which I don't necessarily know what they are. How do I define getter/setter methods on them?"
- bvttf 17y ago
  More appropriately, there's no standard way to do this with CL. One might ask the same about threads. What are threads? We just don't know.
  - mahmud 17y ago
    What do you mean there is no standard way to do this? There is a MetaObject Protocol which is supported by most platforms, and for those that don't have it, there is the widely portable CloserMOP. Threads are not "standard" but the last thread-less Common Lisp just got them this month. Every maintained Common Lisp implementation today has Threads; and all support Bordeaux-Threads. Show me a single mainstream language where threads are "standardized" other than Java?
    - bvttf 17y ago
      You're over-thinking this. My comment was a response to the idea that every lisp can that but there's no single standard. My awkward phrasing was a reference to about 1:53 in this: http://www.youtube.com/watch?v=bCWA7uevo_Q http://www.youtube.com/watch?v=bCWA7uevo_Q
- mahmud 17y ago
  So far there's only one solution: "this can be done (for common lisp) by handling undefined-function condition and invoking appropriate restart (which is unfortunately implementation-specific)." Before you go far and wide on speculation alone, you might stop and reread what has been posted so far, again. Both me and lnostdal submitted working CL solutions. Example of a situation this feature might be useful: "I have this object with lots of properties which I don't necessarily know what they are. How do I define getter/setter methods on them?" ;; if you know the slot names (slot-value object 'slot-name) ==> value (setf (slot-value object 'slot-name) new-value) ;; if you don't. ;; let's define a class hierarchy first CL-USER> (defclass foo () ((a) (b))) #<STANDARD-CLASS FOO> CL-USER> (defclass bar (foo) ((c) (d))) #<STANDARD-CLASS BAR> CL-USER> (defclass baz (bar) ((e) (f))) #<STANDARD-CLASS BAZ> ;; use COMPUTE-SLOTS to get the class's slots CL-USER> (closer-mop:compute-slots (find-class 'baz)) (#<STANDARD-EFFECTIVE-SLOT-DEFINITION A #x1A0F62C5> #<STANDARD-EFFECTIVE-SLOT-DEFINITION B #x1A0F6305> #<STANDARD-EFFECTIVE-SLOT-DEFINITION C #x1A0F6345> #<STANDARD-EFFECTIVE-SLOT-DEFINITION D #x1A0F6385> #<STANDARD-EFFECTIVE-SLOT-DEFINITION E #x1A0F63C5> #<STANDARD-EFFECTIVE-SLOT-DEFINITION F #x1A0F6405>) ;; if you have an object and not a class, do the same CL-USER> (closer-mop:compute-slots (class-of (make-instance 'baz))) (#<STANDARD-EFFECTIVE-SLOT-DEFINITION A #x1A1202A1> #<STANDARD-EFFECTIVE-SLOT-DEFINITION B #x1A1202E1> #<STANDARD-EFFECTIVE-SLOT-DEFINITION C #x1A120321> #<STANDARD-EFFECTIVE-SLOT-DEFINITION D #x1A120361> #<STANDARD-EFFECTIVE-SLOT-DEFINITION E #x1A1203A1> #<STANDARD-EFFECTIVE-SLOT-DEFINITION F #x1A1203E1>)

rsheridan6 17y ago

I don't think so. c[a|d]+r is already too confusing with the builtin functions when there are more than a few [a|d]'s. There are better ways to get at items in a list.