Question

我正在使用Erlang shell来测试语言，并且我用两种方式编写了一个求和函数：一个是更经典的递归和另一个是使用尾递归。第二个是线性的，第一个是随着输入的大小而增长。

如何查看递归的每一步？我希望它在每一步打印出当前值，以便我能更好地理解正在发生的事情。

以下是我的两个功能：

正常递归：

sum([]) -> 0;
sum([H|T]) -> H + sum(T).

尾递归：

tail_sum(List) -> tail_sum(List, 0).
tail_sum([], Sum) -> Sum;
tail_sum([H|T], Sum) -> tail_sum(T, H+Sum).

Answer 1

Erlang中的dbg模块为此提供了一个有用的功能：跟踪。假设您的代码位于模块a：

中，您可以通过以下方式跟踪这些函数

设定：

1> c(a). % load a.erl
{ok,a}
2> dbg:tracer(). % start the default trace message receiver
{ok,<0.71.0>}
3> dbg:p(all, c). % setup call tracing on all processes
{ok,[{matched,nonode@nohost,34}]}
4> dbg:tpl(a, '_', '_', []). % trace all functions in a
{ok,[{matched,nonode@nohost,5}]}

现在无论何时调用模块中的任何函数，它的名称和参数都将打印在shell中。

5> a:sum(lists:seq(1, 10)).
(<0.64.0>) call a:sum([1,2,3,4,5,6,7,8,9,10])
(<0.64.0>) call a:sum([2,3,4,5,6,7,8,9,10])
(<0.64.0>) call a:sum([3,4,5,6,7,8,9,10])
(<0.64.0>) call a:sum([4,5,6,7,8,9,10])
(<0.64.0>) call a:sum([5,6,7,8,9,10])
(<0.64.0>) call a:sum([6,7,8,9,10])
(<0.64.0>) call a:sum([7,8,9,10])
(<0.64.0>) call a:sum("\b\t\n")
(<0.64.0>) call a:sum("\t\n")
(<0.64.0>) call a:sum("\n")
(<0.64.0>) call a:sum([])
55
6> a:tail_sum(lists:seq(1, 10)).
(<0.64.0>) call a:tail_sum([1,2,3,4,5,6,7,8,9,10])
(<0.64.0>) call a:tail_sum([1,2,3,4,5,6,7,8,9,10],0)
(<0.64.0>) call a:tail_sum([2,3,4,5,6,7,8,9,10],1)
(<0.64.0>) call a:tail_sum([3,4,5,6,7,8,9,10],3)
(<0.64.0>) call a:tail_sum([4,5,6,7,8,9,10],6)
(<0.64.0>) call a:tail_sum([5,6,7,8,9,10],10)
(<0.64.0>) call a:tail_sum([6,7,8,9,10],15)
(<0.64.0>) call a:tail_sum([7,8,9,10],21)
(<0.64.0>) call a:tail_sum("\b\t\n",28)
(<0.64.0>) call a:tail_sum("\t\n",36)
(<0.64.0>) call a:tail_sum("\n",45)
(<0.64.0>) call a:tail_sum([],55)
55

您还可以要求跟踪器使用返回return_trace()的匹配规范记录返回的值。这清楚地显示了sum与tail_sum形成对比的增量值7> dbg:tpl(a, '_', '_', [{'_',[],[{return_trace}]}]). (<0.64.0>) call a:module_info() {ok,[{matched,nonode@nohost,5},{saved,1}]} 8> a:sum(lists:seq(1, 10)). (<0.64.0>) call a:sum([1,2,3,4,5,6,7,8,9,10]) (<0.64.0>) call a:sum([2,3,4,5,6,7,8,9,10]) (<0.64.0>) call a:sum([3,4,5,6,7,8,9,10]) (<0.64.0>) call a:sum([4,5,6,7,8,9,10]) (<0.64.0>) call a:sum([5,6,7,8,9,10]) (<0.64.0>) call a:sum([6,7,8,9,10]) (<0.64.0>) call a:sum([7,8,9,10]) (<0.64.0>) call a:sum("\b\t\n") (<0.64.0>) call a:sum("\t\n") (<0.64.0>) call a:sum("\n") (<0.64.0>) call a:sum([]) (<0.64.0>) returned from a:sum/1 -> 0 (<0.64.0>) returned from a:sum/1 -> 10 (<0.64.0>) returned from a:sum/1 -> 19 (<0.64.0>) returned from a:sum/1 -> 27 (<0.64.0>) returned from a:sum/1 -> 34 (<0.64.0>) returned from a:sum/1 -> 40 (<0.64.0>) returned from a:sum/1 -> 45 (<0.64.0>) returned from a:sum/1 -> 49 (<0.64.0>) returned from a:sum/1 -> 52 (<0.64.0>) returned from a:sum/1 -> 54 (<0.64.0>) returned from a:sum/1 -> 55 55 9> a:tail_sum(lists:seq(1, 10)). (<0.64.0>) call a:tail_sum([1,2,3,4,5,6,7,8,9,10]) (<0.64.0>) call a:tail_sum([1,2,3,4,5,6,7,8,9,10],0) (<0.64.0>) call a:tail_sum([2,3,4,5,6,7,8,9,10],1) (<0.64.0>) call a:tail_sum([3,4,5,6,7,8,9,10],3) (<0.64.0>) call a:tail_sum([4,5,6,7,8,9,10],6) (<0.64.0>) call a:tail_sum([5,6,7,8,9,10],10) (<0.64.0>) call a:tail_sum([6,7,8,9,10],15) (<0.64.0>) call a:tail_sum([7,8,9,10],21) (<0.64.0>) call a:tail_sum("\b\t\n",28) (<0.64.0>) call a:tail_sum("\t\n",36) (<0.64.0>) call a:tail_sum("\n",45) (<0.64.0>) call a:tail_sum([],55) (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/2 -> 55 (<0.64.0>) returned from a:tail_sum/1 -> 55 55：

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打印出递归的每一步

1 个答案: