[erlang-questions] Reassigning variables
Ulf Wiger
ulf.wiger@REDACTED
Wed Mar 18 19:37:01 CET 2009
Matthew Dempsky wrote:
>
> Here's an actual block of code from the Erlang compiler:
>
> lc_tq(Line, E, [{generate,Lg,P,G}|Qs0], Mc, St0) ->
> {Gs,Qs1} = splitwith(fun is_guard_test/1, Qs0),
> {Name,St1} = new_fun_name("lc", St0),
> {Head,St2} = new_var(St1),
> {Tname,St3} = new_var_name(St2),
> LA = lineno_anno(Line, St2),
> LAnno = #a{anno=LA},
> Tail = #c_var{anno=LA,name=Tname},
> {Arg,St4} = new_var(St3),
> {Nc,[],St5} = expr({call,Lg,{atom,Lg,Name},[{var,Lg,Tname}]}, St4),
> {Guardc,St6} = lc_guard_tests(Gs, St5), %These are always flat!
> {Lc,Lps,St7} = lc_tq(Line, E, Qs1, Nc, St6),
> {Pc,St8} = list_gen_pattern(P, Line, St7),
> {Gc,Gps,St9} = safe(G, St8), %Will be a
> function argument!
> Fc = function_clause([Arg], LA, {Name,1}),
> ....
[...]
> But, please, show me how you would write v3_core:lc_tq/5 without
> numbered state variables without losing any readability.
So there's a pattern there, and there's actually a library
function that might help to eliminate most of the numbered
variable names.
thread(Fs, St) ->
lists:mapfoldl(fun(F,St1) -> F(St) end, St, Fs).
lc_tq(Line, E, [{generate,Lg,P,G}|Qs0], Mc, St0) ->
{Gs,Qs1} = splitwith(fun is_guard_test/1, Qs0),
{[Name, Head, Tname], St1} =
thread([fun(St) -> new_fun_name("lc", St) end,
fun new_var/1,
fun new_var_name/1], St0),
...
Whether it's more readable is perhaps a matter of
taste. It ought to be easier to maintain though.
BR,
Ulf W
--
Ulf Wiger
CTO, Erlang Training & Consulting Ltd
http://www.erlang-consulting.com
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