[erlang-questions] re cursive fun()

Edwin Fine <>
Sun Oct 5 17:03:49 CEST 2008


Just noticed a mistake.

FunVar =
   fun(MySelf) ->
      ...
      MySelf().

should have been

FunVar =
   fun(MySelf) ->
      ...
      MySelf(MySelf).

On Sun, Oct 5, 2008 at 5:13 AM, Edwin Fine
<>wrote:

> I'm assuming that you want to do this in the shell, otherwise it would be
> easier just to write the function in the usual way, e.g.
>
> myfun() ->
>     blah, blah,
>     myfun().
>
> To create a fun that calls itself, you must realize that you are creating
> an unnamed function. When you write
>
> fun(X) -> X * 2 end.
>
> this is compiled to a Fun data type, e.g. #Fun<erl_eval.6.13229925>.
>
> Since the function does not have a name, it cannot refer to itself.
>
> When you write FunVar = fun(X) -> X * 2 end, the function is bound to the
> variable FunVar. Because the binding takes place *after* the fun is created,
> the fun cannot refer to the variable FunVar within its definition.
>
> So how is it done? You have to pass the variable to the function so that it
> "knows" what to call:
>
> FunVar =
>    fun(MySelf) ->
>       ...
>       MySelf().
>
> Then when you call it, you call FunVar(FunVar). Looks strange but it works.
>
> Example: Writing a simple generate and fold function using only a fun()
> variable.
>
> 9> GenFold =
>    fun(0, Acc, _) ->
>       Acc;
>    (X, Acc, Fun) ->
>       Fun(X - 1, [X|Acc], Fun)
>    end.
> #Fun<erl_eval.18.105910772>
> 10> GenFold(10, [],
> GenFold).
> [1,2,3,4,5,6,7,8,9,10]
> 11>
>
> Or a for loop:
>
> 22> For =
>     fun(Max, Max, ExecFun, _) ->
>        ok;
>    (N, Max, ExecFun, Next) ->
>       ExecFun(N),
>       Next(N + 1, Max, ExecFun, Next)
>    end.
> #Fun<erl_eval.4.105156089>
> 23> For(0,10,fun(I) -> io:format("~B ", [I]) end,
> For).
>
> 0 1 2 3 4 5 6 7 8 9 ok
> 24>
>
> Stuff like this is described in Joe Armstrong's Erlang book.
>
> Hope this helps.
>
>
> On Sat, Oct 4, 2008 at 11:19 PM, deepblue_other <> wrote:
>
>>
>> hello
>> I have this going on
>>
>> FunVar =
>>   fun() ->
>>      ...
>>      FunVar()
>>   end.
>>
>> so the compiler is complaining that FunVar is unbound at the place where
>> its
>> being used inside fun(); this makes sense, however Im wondering how to
>> make
>> this into a recursive function since there's no name to reference the
>> function with.
>>
>> thanks
>> --
>> View this message in context:
>> http://www.nabble.com/recursive-fun%28%29-tp19820386p19820386.html
>> Sent from the Erlang Questions mailing list archive at Nabble.com.
>>
>> _______________________________________________
>> erlang-questions mailing list
>> 
>> http://www.erlang.org/mailman/listinfo/erlang-questions
>>
>>
>
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