Access is a common lisp library I just culled out of our immense utility mud ball and refactored into a library all its own. Access makes getting and setting values in common data structures support a single unified api. As such you could access a specific key from an alist stored in a hashtable stored in the slot of an object as (accesses o ‘k1 ‘k2 ‘k3). It also supports setting values (setf (accesses o ‘k1 ‘k2 ‘k3) “new-val”). Obviously there are some limitations to this approach, but for me, with my coding conventions, I don’t tend to run into them (see the README for details).

Access has removed some of my need for forms like (awhen a (awhen (fn1 it) (fn2 it))) with (access a ‘fn1 ‘fn2). To me, it allows me to more accurately express what I am trying to do while ignoring the vagaries of shifting implementation details. It also eases setting values in nested objects because it handles propagating the value up the chain rather than me having to do that myself (ie adding a new key-value pair to a the front of an alist stored in an object, automatically saves the new resulting alist in the object). I don’t expect that this is tasteful coding, but it is easier and allows me to not get mired down trying to decide if I want it to be an alist, plist, hashtable, or object because the cost to change it later is essentially zero.

Performance is rarely in issue in the apps that I tend to write. However, if it were, I would not use access as it does significant type and dispatch analysis that could be avoided by using the specific access functions of the data structure I am using.

A dot syntax familiar to those who use javascript/python/ruby type languages is available as well. This transforms calls like foo.bar.bast into (accesses foo ‘bar ‘bast). I don’t use this syntax as I tend to prefer the lisp function-call syntax, but it seems to be an oft requested / discussed feature, and I had fun writing the code.