[erlang-questions] help with message-passing syntax

Richard A. O'Keefe ok@REDACTED
Fri Jul 4 04:15:53 CEST 2008

On 3 Jul 2008, at 9:04 pm, not norwegian swede wrote:
> squarer(X) ->
>     receive
>     Pattern [when Pattern > 7] ->
>         [X*X || X <- [seq(1, 7]];
>     end

Why the square brackets around the guard?

You are receiving Pattern, and checking it,
but then you do nothing with it!
Pattern is a bad name because it tells us nothing
whatever about the kind of messages you are expecting
to receive.

You have an argument X, and then you appear to be
trying to bind a *different* X in the list comprehension.

Your list comprehension

	[X*X || X <- [seq(1, 7)]]

has an extra pair of square brackets, for no apparent
reason.  The list [seq(1, 7)]] has only one element,
so X will be bound (just once) to [1,2,3,4,5,6,7].
However, [1,2,3,4,5,6,7]*[1,2,3,4,5,6,7] is not understood.
Presumably you meant

	[X*X || X <- seq(1, 7)]

However, having computed the list [1,4,9,...,49], what do
you want to do with it?

I would expect a squaring process to be something like

	squarer(Destination) ->
	    receive Number ->
		Destination ! (Number * Number),

> squarer2(X) ->
>     receive
>     when X > 7 -> [X*X || X <- [seq(1, 7]];
>     end

This has two of the same defects as the previous version:
X used as argument and as list element pattern,
extra brackets around seq(1, 7).
It adds a new one: there is no pattern in your 'receive'
saying _what_ you want to receive or letting you remember
what it is.
> squarer(X) ! 2+6

This will call the *FUNCTION* squarer, in the expectation
that it will return a process ID.  The function squarer()
will enter the 'receive', and wait forever for a message.

I think you mean something like this:

	destination() ->
		stop ->
	      ; Number ->
		    io:write(Number), io:nl(),

	squarer(Destination) ->
		stop ->
		    Destination ! stop
	      ; Number ->
		    Destination ! (Number * Number),

	sender(Squarer, Numbers) ->
	    [Squarer ! Number || Number <- Numbers],
	    Squarer ! stop.

	example() ->
	    Destination = spawn(fun () -> destination() end),
	    Squarer = spawn(fun () -> squarer(Destination) end),
	    sender(Squarer, seq(1, 7)).


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