[Haskell-cafe] Haskell vs GC'd imperative languages, threading, parallelizeability (is that a word? :-D )

Hugh Perkins hughperkins at gmail.com
Fri Aug 10 04:58:36 EDT 2007

On 8/10/07, Michael Vanier <mvanier at cs.caltech.edu> wrote:
> Just to get the history right: garbage collectors have been around a
> _long_ time, since the '60s in
> Lisp systems.  They only became known to most programmers through Java
> (which is one unarguable good
> thing that Java did).

Ah interesting :-)

> As for threading, in addition to Haskell's approach you might also look at
> Erlang, which has a quite
> different (and quite interesting) approach to the whole problem.  I wonder
> if anyone has tried to
> implement a message-passing style of concurrency in Haskell.

Erlang message passing rocks :-)  I'd love to see this working in Haskell.

Note that Erlang's message passing is not perfect.  Specifically, there is
zero type or parameter checking.  It will quite happily let the following

% initialization routines etc go here
% ...

% ping thread. this sends messages to pong
ping() ->
   pong ! hello,
   pong ! hello,
   pong ! {hello},   % this does not throw a compile error
   pong ! hello,
   io:format("ping done~n" ).

% pong thread, this listens for messages from pong
pong() ->
      hello ->
         io:format("pong received hello~n"),

You can see that sending the tuple {hello} does not cause a compile time
error (or even a runtime error for that matter), even though pong does not
have a valid pattern for receiving it.

Now, arguably the fact that we are pattern matching on the receiver at least
means we dont do anything with the invalid data sent, but this is not rocket
science: the standard technique to ensure decent compile time validation in
rpc-type things is to use an interface.

The interface defines the method names and parameters that you can send
across.  Both the receiver and the sender have access to the interface
definition, and it is trivial to check it at compile time.

(Caveat: havent looked enough into Erlang to know if there is a good reason
for not using an interface?)
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