[erlang-questions] erlang *****

Andras Georgy Bekes bekesa@REDACTED
Tue Mar 18 13:54:19 CET 2008

> However, if pattern matchings are done one-by-one and left-to-right,
> then there is no performance problem. The union operator becomes less
> flexible in this case, but it is still quite useful.

> Actually, if we want the pattern matching to be deterministic, we
> _must_ do such a restriction. For example using the patter {X,_} V
> {_X}, the value of X could be either element of the tuple. Using the
> left-to-right rule, X will always be the first element.


My proposed pattern match -- with union -- is always deterministic. It 
uses the first matching pattern found by a left-to-right 

In the above example, {X,_} \/ {_,X} always matches X with the first 
element. But matching {{a,b},b} with { {X,_} \/ {_,X}, X} matches the 
second. It tries {X,_} first, succeeds, then tries matching b with a. 
It fails, so backtracks and tries {_,X}.

Your proposed "one-by-one and left-to-right" method would fail when 
matching {{a,b},b} with { {X,_} \/ {_,X}, X}.

They're completely different.


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