[erlang-questions] per function/process locals

[Ulf Wiger (TN/EAB)]

I could have mentioned, for completeness, that the
two alternatives are not semantically equivalent,
if the list holds more than one instance of any
given key.

Using proplists, you take the first instance of K,
whereas with the foldl version, you will keep the 
last. It can often be useful to allow shadowing of
options, and with proplists, this is done by 
prepending to the list.

I will read your frames proposal.

BR,
Ulf W

> -----Original Message-----
> From: erlang-questions-bounces@REDACTED 
> [mailto:erlang-questions-bounces@REDACTED] On Behalf Of ok
> Sent: den 3 maj 2007 02:56
> To: Erlang (E-mail)
> Subject: Re: [erlang-questions] per function/process locals
> 
> On 2 May 2007, at 11:21 pm, Ulf Wiger (TN/EAB) wrote [that 
> instead of doing]
> > init_state(L) ->
> >   lists:foldl(
> >     fun({a, A}, S) -> S#st{a = A};
> >        ({b, B}, S) -> S#st{b = B};
> >        ...
> >     end, #st{}, L).
> [he tends to do]
> >   [A, B, C, D] =
> >     [proplists:get_value(K,L) ||
> >       K <- [a, b, c, d]],
> >   #st{a = A, b = B, c = C, d = D}
> 
> This is a rather interesting example, because it shows a 
> clear trade- off.
> Suppose there are V "variables".  We expect that the list 
> will contain each at most once; the worst case is when they 
> all appear.
> 
> The first algorithm does one pass -- O(V) work for traversal 
> -- building a new record (memory size V+2 words at least) on 
> each iteration, for a total of O(V**2) time and O(V**2) space 
> turned over.
> The second algorithm does V passes -- O(V**2) work for 
> traversal -- but each pass builds only a single cons cell -- 
> O(V) space --.  For large V, it is better to sort the list L 
> -- O(V.lg V) time and space -- and then do a merge -- O(V) 
> time and space --.  This is basically what
> list_to_frame/1 does in my "frames" proposal (which is not 
> entirely unlike Joe Armstrong's "proper structs").  See 
> http://www.cs.otago.ac.nz/staffpriv/ok/frames.pdf
> for details.
> 
> It would be interesting to see more examples where people want "many"
> "variables" in a loop, to discover whether they could be 
> handled in the source language by multiple loops each working 
> with fewer variables.
> If so, compiler support for loop fusion (which would be a 
> very nice thing to have anyway) might do more for us than 
> special syntax.
> 
> 
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