[erlang-questions] re cursive fun()

Steven Grady <>
Tue Oct 7 03:06:55 CEST 2008


BTW, this question has apparently come up before (multiple times).   
The key google phrase is "Y combinator":
	http://bc.tech.coop/blog/070611.html

	Steven

On Oct 5, 2008, at 2:13 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
>
>
> _______________________________________________
> erlang-questions mailing list
> 
> http://www.erlang.org/mailman/listinfo/erlang-questions

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20081006/898be408/attachment.html>


More information about the erlang-questions mailing list