[erlang-questions] Basic question about Erlang binaries

Robert Virding rvirding@REDACTED
Sat Sep 26 22:04:58 CEST 2009

2009/9/26 Igor Ribeiro Sucupira <igorrs@REDACTED>

> >
> > This is not really strange if you consider what you are asking it to do.
> You
> > are telling it to build a 1048575 bit integer,
> Am I?
> Maybe I'm missing some concept, but to me it seems I'm asking it to
> build Bit and to make sure the pattern matching is valid. So, what is
> necessary is to check that the integer can be built and is unique.
> Can't this be done in constant time?
> In other words, I believe the pattern matching above means something
> like: "Can you please give me the 1048576th bit, as long as the first
> 1048575 bits represent a unique (signed, little-endian, blablabla)
> integer and and the rest represent an Erlang binary?"
> I'm being pretty informal, as it's Saturday (:-)), but I believe you
> understand my point.

Yes I understand your point. :-)

The problem is that what you MEAN is give me the 1048576th bit, but what you
WRITE in your code is: take the first 1048575 bits and make an integer, then
throw it away; take 1 bit and return it; put the remaining bits into a
binary and then throw it away.

This may be a bit on the philosophical side but pattern matching is really
pulling a structure apart into its components and not really accessing a
small part of a structure, although it is often used that way. As in this
case. That is why you have to give a pattern for the whole binary. Or so I
have always considered it.

Now the interpreter is pretty naive and does exactly what you tell it, but
the compiler should be better, though I haven't tried it.


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