[erlang-questions] How to make this work

[Rick Pettit]

Not sure this is *exactly* what you are after, but…

-module(my_time).
-export([my_time_func/2]).

my_time_func(F, Arg) ->
  T0 = erlang:now(),
  io:format("Fun returned: ~p~n", [F(Arg)]),
  T1 = erlang:now(),
  Elapsed = timer:now_diff(T1, T0),
  io:format("Elapsed time: ~p microseconds~n", [Elapsed]).


===

rpettit-ltm:~ rpettit$ erl
Erlang/OTP 17 [erts-6.4] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

Eshell V6.4  (abort with ^G)
1> c(my_time).
{ok,my_time}
2> my_time:my_time_func(fun(X) -> 2 * X end, 3).
Fun returned: 6
Elapsed time: 73 microseconds
ok

===

Note that erlang:now/0 is deprecated, the above was just to illustrate one way to work with that fun you are having trouble with.

Again, the above may not *exactly* solve your problem, but the code works and you can feel free to play with it until you are comfortable with what is going on there.

My recommendation would be to use an iterative approach—take something which works, make very small changes and continually test. This way when you break something, you immediately know where to look.

When starting from scratch, build something *small*, make it work, then slowly add to it all the while going back to run tests, verify you didn’t introduce any bugs, etc.

-Rick

> On Aug 12, 2015, at 10:51 AM, Roelof Wobben <r.wobben@REDACTED> wrote:
> 
> I think I have a too old version because I see this :
> 
> exception error: undefined function erlang:monotonic_time/0 in function my_time:time_spend/1 (my_time.erl, line 6)
> 
> I have R16B03,
> 
> Roelof
> 
> 
> 
> Op 12-8-2015 om 17:47 schreef Roelof Wobben:
>> Correct but my question is how I can access the function with a fun.
>> 
>> Roelof
>> 
>> 
>> Op 12-8-2015 om 17:39 schreef Dmitry Kolesnikov:
>>> Hello,
>>> 
>>> You can use built-in function
>>> http://erldocs.com/17.0/stdlib/timer.html?i=0&search=timer:tc#tc/1
>>> 
>>> You can also check from OTP source the implementation of this function and compare it with your's
>>> https://github.com/erlang/otp/blob/maint/lib/stdlib/src/timer.erl#L160
>>> 
>>> It gives you hints :-)
>>> 
>>> Best Regards,
>>> Dmitry
>>> 
>>>> On 12 Aug 2015, at 18:34, Roelof Wobben <r.wobben@REDACTED> wrote:
>>>> 
>>>> Hello,
>>>> 
>>>> Im trying this exercise from the programming erlang book.
>>>> 
>>>> Look up the definitions of erlang:now/0, erlang:date/0, and erlang:time/0. Write a
>>>> function called my_time_func(F), which evaluates the fun F and times how
>>>> long it takes.
>>>> 
>>>> So I did this :
>>>> 
>>>> -module(my_time).
>>>> 
>>>> -export( [time_spend/1] ).
>>>> 
>>>> time_spend(F) ->
>>>>    Begintime = time:now(),
>>>>    F(3),
>>>>    Endtime = time:now(),
>>>>    Endtime - Begintime.
>>>> 
>>>> 
>>>> but then when I do this in erl :
>>>> 
>>>> 11> my_time:time_spend(fun x -> 2 * X end).
>>>> * 1: syntax error before: '->'
>>>> 11> my_time:time_spend(Double = fun x -> 2 * X end).
>>>> * 1: syntax error before: '->'
>>>> 
>>>> how can I make this work ?
>>>> 
>>>> Roelof
>>>> 
>>>> 
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