[erlang-questions] eep: multiple patterns

[Hynek Vychodil]

+1

On Thu, May 15, 2008 at 3:27 AM, Richard A. O'Keefe <ok@REDACTED>
wrote:

> I feel that this proposal is a step in the wrong direction,
> and has "hack" written all over it.
>
> Let's start with the pattern-match operator,
>
>        pattern ~= expression
>
> I already feel slightly queasy at the fact that pattern matching
> is associated with guards everywhere EXCEPT pattern = expression;
> adding another construction that will work some but only some of
> the time does not appeal to me.  Right now, I can do
>
>        1 = f()
>
> but I cannot do
>
>        (X when X > 1) = f()
>
> Right now, any instance of
>        case E1 of P1 -> B1 end         % just one clause!
> can be rewritten as
>        (P1 = E1, B1).
> Why not extend this to allow
>        case E1 of P1 when G1 -> B1 end
> to be written as
>        ((P1 when G1) = E1, B1)
>
> I'm not pushing _hard_ for this, you understand, just
> noting that there is a nasty and apparently unmotivated
> gap in the language, and suggesting that something
> should be done about it.  Abstract patterns are, of
> course, a far better solution to the same issue.
>
> Of course the same idea can be extended to ~=, and if ~=
> were to be extended, it should be.
>
> The draft EEP notes that the ~= construction adds no
> real power (and, if we consider macros, no extra power
> at all) in ordinary expressions.  It is ONLY useful in
> guards.  But WHY is it useful?
>
> ~= is only useful because = is not allowed in guards.
>
> Between a pattern match test that doesn't bind any
> variables and a pattern match test that only binds
> variables that happen not to be used anywhere else
> there is no real difference.  ~= is a special case of
> = .  So if we are willing to consider ~= in guards,
> why not consider = in guards?
>
> Indeed, why not?  If I can write
>
>        f(L) when hd(L) > 0, hd(L) < 10 ->
>
> why can't I write
>
>        f(L) when N = hd(L), N > 0, N < 10 ->
>
> In the past I have strongly defended the distinction
> between guards and expressions, and I have not changed
> my mind.  But allowing = in guards does not destroy any
> of the properties of guards that have ever concerned me.
>
> So instead of adding a new "crippled-bind" called ~=,
> I suggest that it would be better to allow
> Pattern = Expression as a guard.
>
> What about the intended uses of ~= where the fact that
> it does not bind variables is important?  Why then,
> just restrict the corresponding = to not bind variables.
> {foo,_,27} = hd(X) would have the same effect in a guard
> as {foo,_,27} ~= hd(X) would have, so if we allowed =,
> there would be no point in having ~= as well, while if
> we had ~= we would still be left lamenting the
> inexplicable prohibition on =.
>
> Do I need to mention that the similarity between = and
> ~= would lead to errors when one was used but the other
> intended?  No, I thought not.  ~= is something we are
> better off without.  But its use in guards _is_ something
> we could do with, only the existing = would be far better.
>
> That's enough for one message, I think.
>
> Maybe not.
>
> I don't like vague suggestions that
> "It is rather common to check the value of an expression with a
>     case expression and do the same actions in some of the cases."
> I want to see *actual cases*.
>
> Oh, and did I point out that abstract patterns can solve the
> "multiple patterns" problem neatly and clearly?
>
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-- 
--Hynek (Pichi) Vychodil
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