[erlang-questions] List to proplist?

[Richard O'Keefe]

On 20 Oct 2008, at 5:56 pm, Edwin Fine wrote:
> I know it would be easy to write a pattern-matching-style function,  
> but I wanted to do it without having to define a new function,
>
>     element(1, lists:mapfoldl(fun(_,[X,Y|T]) -> {{X,Y}, T} end, L,  
> lists:seq(1, length(L) div 2))).

But you just defined a new function and a pattern-matching one
at that.

The obvious function is

to_proplist([Key,Val|Rest]) ->
     [{Key,Val} | to_proplist(Rest)];
to_proplist([]) ->
     [].

Now let's pretend that you've heard of "unfold",
maybe even read
http://debasishg.blogspot.com/2007/11/fun-with-unfold-in-erlang.html

Let's copy the unfold function from there.

unfold(Seed, Predicate, Transformer, Incrementor) ->
     unfold(Seed, Predicate, Transformer, Incrementor, []).

unfold(Seed, Predicate, Transformer, Incrementor, Acc) ->
     case Predicate(Seed)
       of false -> lists:reverse(Acc)
        ; true  -> unfold(Incrementor(Seed),
                          Predicate,
                          Transformer,
                          Incrementor,
                          [Transformer(Seed)|Acc])
     end.

Now it's fairly straightforward:

to_proplist(Even_List) ->
     unfold(Even_List,
            fun (L)           -> L =/= []  end,
            fun ([Key,Val|_]) -> {Key,Val} end,
            fun ([_,_|Rest])  -> Rest      end).

When you want to generate a list, an unfold is often what
you want rather than a fold.

I don't suppose we could have unfold/4 in the lists module,
could we?