Feature #12752: Unpacking a value from a binary requires additional '.first'

• Status: Closed
• Assignee: -
• Target version: -

[tagomoris (Satoshi Tagomori)]

When I parsed a binary protocol (msgpack) by String#unpack, there are many similar expressions, like below:

str = io.read(1)
length = str.unpack('C').first
case length
when 4
  data = io.read(4).unpack('N').first
# ...

I know that unpack returns "1 or more" values, but I think there are too many cases to return just one object.
Creating Array objects for return values requires additional cost to create Array object, and calling .first.

I'm happy if I can write the code above like this:

str = io.read(1)
length = str.unpack('C', 0) # index, or str.unpack('C', index: 0)
case length
when 4
  data = io.read(4).unpack('N', 0)
# ...

[shyouhei (Shyouhei Urabe)]

• Description updated (diff)

[mame (Yusuke Endoh)]

There is more concise way:

length, = str.unpack('C')

You cannot use this in a method chain, though.

I actually understand your frustration, but I feel no difference between str.unpack('C', 0) and str.unpack('C')[0].

--
Yusuke Endoh [e-mail]

[naruse (Yui NARUSE)]

How about Array#unpack1 ?

[matz (Yukihiro Matsumoto)]

unpack1 looks so ad-hoc but surely is attractive.

Matz.

[matz (Yukihiro Matsumoto)]

Go ahead.

Matz.

[knu (Akinori MUSHA)]

With a qualifier or multiple directives given, would it raise an ArgumentError or just go ahead with the process and only return the first value?

[naruse (Yui NARUSE)]

Akinori MUSHA wrote:

With a qualifier or multiple directives given, would it raise an ArgumentError or just go ahead with the process and only return the first value?

Just after processing the first value, it returns.

[naruse (Yui NARUSE)]

• Status changed from Open to Closed

Applied in changeset r56959.

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String#unpack1 [Feature #12752]

Returns the first value of String#unpack.