[erlang-questions] tail recursion

[Joe Armstrong]

On 5/31/07, Ulf Wiger <ulf@REDACTED> wrote:
>
>
> 2007/5/31, Jason Dusek <jsnx@REDACTED>:
> >
> > Its tail recursive but, unfortunately, a little redundant -- I'd like
> > to rewrite it like this:
> >
> > server({X, Y}) ->
> >   receive
> >     {update, NewState} ->
> >       server(NewState);
> >     {x, Client} ->
> >       Client ! X;
> >     {y, Client} ->
> >       Client ! Y;
> >   end,
> >   server({X, Y})
> >   .
> >
> > But I worry that the compiler and interpreter will fail to recognize
> > my server as tail recursive. Will my rewrite blow the stack?
> >
>
> Yup, since the call to server/1 from within the receive clause is not in
> fact a tail call.
>
> A common way to solve this dilemma is to do this:
>
> server({X, Y} = State) ->
>     S1 =
>         receive
>             {update, NewState} ->
>                 NewState;
>             {x, Client} ->
>                 Client ! X,
>                 State;
>             {y, Client} ->
>                 Client ! Y,
>                 S
>         end,
>     server(S1).
>
> I added the "= State" in the function head so I wouldn't have to rebuild the
> tuple when nothing has changed.
>

Why not just

  server({X, Y} = State) ->
     receive
             {update, NewState} ->
                 NewState;
             {x, Client} ->
                 Client ! X,
                 server(State);
             {y, Client} ->
                 Client ! Y,
                 sever(State)
    end.

/Joe

> BR,
> Ulf W
>
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