[erlang-questions] Dictionary strategy of last value wins

Bob Ippolito <>
Sun Jun 22 20:16:44 CEST 2014


If you do want to keep a full history of inserts, couldn't you use a
dict(K, [V])? Sure, you'd have to write a few functions, since
dict:append/3 doesn't have the desired semantics, but such functions are
straightforward to write.

prepend(K, V, D) ->
    dict:update(K, fun (Vs) -> [V | Vs] end, [V], D).


On Sun, Jun 22, 2014 at 11:06 AM, Jay Nelson <> wrote:

> I’m implementing a library using dictionaries and I’ve noticed that all the
> dictionary alternatives use imperative-style initialization semantics.
> Proplists were
> introduced to allow efficient shadowing of environments and defaults.
> Pushing a value on the front of a proplist makes the first value
> encountered
> be the current value. You can then pop contexts and reveal the older values
> efficiently in a functional programming style.
>
> Dictionaries use hash-like semantics of last value wins in an imperative
> assignment that clobbers old values (and loses them forever) thus making
> them non-functional and non-persistent. I assume initialization of
> dictionaries
> are kept imperative for consistency / efficiency, but it makes the two
> styles
> of initialization incompatible (but keeps the internal semantics of each
> datatype consistent).
>
> The recommended technique for maps is to set a default dictionary and
> then the shadowing dictionary and merge them. I guess this leads to a
> functional style of a stack of maps for shadowing contexts, but this
> approach
> is only recommended as overcoming default initialization. In recent code,
> I’ve had to use different semantics for defaulted values vs user-specified
> values and having a dictionary meant I couldn’t tell whether the current
> value was an implicit default or an explicit choice after the
> initialization
> phase had completed.
>
> Do others build proplists and then reverse them when initializing a
> dictionary?
> (This seems very imperative, having to maintain your own data versions:
> proplist and dictionary init list, especially if you want shadowing
> semantics
> and need to go back to the source context and initialize subsequent
> dictionaries).
>
> Or do you maintain a stack of dictionaries and do a cascading search until
> a value is found? This approach allows context labeling and thus you could
> know if a value was explicit or implicitly defaulted.
>
> Or do you just change to an imperative style for your code when you start
> thinking with dictionaries (or have you not noticed the difference)?
>
> My new library is supposed to allow selection of implementation without
> change to the higher-level logic. I suppose I am going to have to provide
> ‘proplist (functional)’ and ‘last wins (imperative)’ options for
> initialization
> of the underlying implementation (and do proper initialization in all
> cases).
>
> My current approach is to stick with a functional style of programming
> and use proplist semantics on initialization of my object wrappers around
> dictionaries (dict, orddict, vbisect; maps in future; possibly will add
> proplists and records as options as well, and I suppose arrays could
> be used). The current overhead expense is that I have to scan the init
> list while constructing the dictionary doing lookups to see if a value is
> already set and skip it in the case of an earlier definition (I also do
> this
> because I sometimes need to validate the datatypes to ensure that
> constructed dictionaries use legal types on all input — I could switch
> to a faster initialization in cases where datatype doesn’t matter by
> reversing the list, assuming the list size is never ginormous).
>
> jay
>
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