access

2014-08-22

Access

A Common Lisp library to unify access to the most common data structures and to allow you to operate on them as they are (ie as a bunch of dictionaries with slightly different apis)

access, accesses , (setf access), (setf accesses)

These functions allow unified access to these data structures:

  • accessor access to CLOS objects
  • slot access to CLOS objects if the key matches a slot name but not an accessor
  • plists
  • alists
  • hash-tables
  • arrays

They also opts to produce nil as opposed to signaling errors when they fail to access (eg (access nil 'anything) produces nil rather than signaling a missing method on nil (though if 'anything is specialized on nil it will call that method)). Slot unboundedness errors are not signaled.

This library will probably appeal most to new comers to the language as everyone else will probably be happy just calling each type of access according to its own api.

accesses, (setf accesses)

These can be handy for modifying deeply nested structures without lots of intermediary bindings eg:

(setf (accesses ucw::context 'ucw::context.request ucw::parameters '("id" :type :alist)) 2043)

Will correctly set the "id" parameter of the request to 2043. It will not signal an error if request is context is unbound, nor any of the slots.

The '("id" :type :alist) is required because ucw expects an alist, but access will default to plist when asked to set on a nil.

do-access, do-set-access

When we fail to find an reader/writer function, access will ultimately have to be reading and writing a datastructure. That happens in these generic functions. These functions also allow access extensibility to support any conceivable map datastructure.

What happens when setting through nil?

Access will create a dictionary to put stuff into. The type of dictionary will depend on the :type parameter.

  • :type is expected to be: cl:array, :array, cl:hash-table , :plist, :alist, :object, cl:standard-object, or a type that we can call 'make-instance on with no arguments.

EX:

=>(setf (accesses place
         '(:a :type :alist)
         '(2 :type array)
         '(:b :type 'hash-table)) 3)
;; 3

=> place

;; ((:a . #(nil nil #<hash-table :b=3 >)))

Limitations

  • Accessors should share slot names for this to work best. This is due to differences in "direct" class slots versus indirect slots (only direct slots have the reader value filed out).
  • While most structures use equalper to get around differnt key packages and strings vs symbols. Hash-tables do not currently support an equalper style interface. As such some small care needs to be taken. We try to support this by looking up values by symbol, then by symbol-name if symbol fails to produce a result.

A word on performance

This libary is meant to make writing the program easier. It does many runtime lookups and checks to make sure that funcations called can support the types they are called with. As such it should not be used in code where performance is important. It should however allow you to prototype more rapidly and change the backing data stores without having to change their access (ie I can switch from a plist to an alist and everything will continue to work)

Utilities

has-slot?, has-reader?, has-writer?

Given a function or symbol, see if the object has a slot named that or a reader/writer function associated with that name

class-slot-names, class-slot-readers, class-slot-writers

Returns the names associated with the classes slots. Readers and writers returns the functions used to access and set these slots, however these currently only support readers/writers with the same name as the slot.

call-if-applicable, call-applicable-fns

Given an object and a function / funcation-name, this will call the function passing in the object if it seems like that will work

class-of-object

A helper to find you the class of a given thing

  (typecase o
      (symbol (find-class o))
      (standard-class o)
      (standard-object (class-of o)))

equalper

A predicate to make comparing symbols in different packages easier, by comparing them case-insensitively based on symbol-name. In other respects it is equalp.

plist-val, rem-plist-val, set-plist-val

Functions to ease access to plist values (used by access when detecting a plist)

DOT syntax

DOT syntax is invoked with #D reader macro on a form or by wrapping that form in a with-dot call

Many new-comers to the language long for their dot operator from other lanugages they know. This functionality is provided (when desired) by enable-dot-syntax (for #D) or wrapping a block in the with-dot macro. I wrote these for fun and much prefer just using the access functions directly (ie. I never actually use these syntax transformers). That said, when the dot syntax is enabled, symbols with a dot in them will be transformed to the appropriate accesses calls.

EX: #Dfoo.bar.bast => (accesses foo 'bar 'bast) EX: (with-dot () (setf ht.key.subkey new-val)) => (setf (accesses ht 'key 'subkey) new-val)

Authors

;; Copyright (c) 2011 Russ Tyndall , Acceleration.net http://www.acceleration.net
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