[erlang-questions] Name For This Pattern?
David Mercer
<>
Tue Aug 21 00:46:43 CEST 2007
OK, so I want to generate a sequence based on some mathematical formula.
For instance, [1, 2, 3, 4, 5, .] or [1, 2, 4, 8, 16, .]. Since these are
infinite, I cannot implement them as lists. Instead, I implement them as a
function:
2> % [1, 2, 3, 4, 5, ...]
2> F0a = FF(fun(X) -> X + 1 end, 1).
#Fun<erl_eval.20.112921583>
3> {_, F1a} = F0a().
{1,#Fun<erl_eval.20.112921583>}
4> {_, F2a} = F1a().
{2,#Fun<erl_eval.20.112921583>}
5> {_, F3a} = F2a().
{3,#Fun<erl_eval.20.112921583>}
6> {_, F4a} = F3a().
{4,#Fun<erl_eval.20.112921583>}
7> {_, F5a} = F4a().
{5,#Fun<erl_eval.20.112921583>}
12> % [1, 2, 4, 8, 16, ...]
12> F0b = FF(fun(X) -> 2 * X end, 1).
#Fun<erl_eval.20.112921583>
13> {_, F1b} = F0b().
{1,#Fun<erl_eval.20.112921583>}
14> {_, F2b} = F1b().
{2,#Fun<erl_eval.20.112921583>}
15> {_, F3b} = F2b().
{4,#Fun<erl_eval.20.112921583>}
16> {_, F4b} = F3b().
{8,#Fun<erl_eval.20.112921583>}
17> {_, F5b} = F4b().
{16,#Fun<erl_eval.20.112921583>}
I am sure many of you have done this before, and I haven't discovered
anything new, but I wanted to figure it out for myself, so I did. The
function I came up with was:
FF = fun(F, X0) ->
G = fun(F, X0, G) ->
{ X0, fun() -> G(F, F(X0), G) end }
end,
fun() -> G(F, X0, G) end
end.
1. Is this the way you'd have done this?
2. Is there a name for this pattern? I was calling it a continuation
in my mind, because the second element of the output 2-tuple is a
continuation function to give the next value, but I thought maybe there is a
more specific name for this use of continuations.
Cheers,
David
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