bytes.Writer?

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Mitchell Hashimoto

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Aug 17, 2013, 2:03:28 PM8/17/13
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Hello,

There is a bytes.Reader for turning a raw []byte into an io.Reader. Why is there no equivalent for io.Writer? I suppose I could use "bytes.Buffer" but I don't want it to grow. 

The reasoning is I want to copy an io.Reader where I know the length (so io.CopyN) into a []byte. Thoughts?

Best,
Mitchell

Ludwig Valda Vasquez

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Aug 17, 2013, 2:15:14 PM8/17/13
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Why not just use Read method of io.Reader?

http://play.golang.org/p/8ebDizKbr2

суббота, 17 августа 2013 г., 22:03:28 UTC+4 пользователь Mitchell Hashimoto написал:

Ian Lance Taylor

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Aug 17, 2013, 2:16:06 PM8/17/13
to Mitchell Hashimoto, golang-nuts
Pass your slice to bytes.NewBuffer. If you don't pass too many bytes,
it won't grow.

I don't think there are any length-restricted writers in the standard
library, as it's not a feature many people need. If there is a clear
use case for bytes.Writer or io.LimitWriter I suppose they could be
added, where by a clear use case I don't mean "I can write a program
that uses them" but "this important program or protocol would benefit
from them in this way."

Ian

Benjamin Measures

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Aug 17, 2013, 6:49:33 PM8/17/13
to golan...@googlegroups.com
On Saturday, 17 August 2013 19:03:28 UTC+1, Mitchell Hashimoto wrote:
There is a bytes.Reader for turning a raw []byte into an io.Reader. Why is there no equivalent for io.Writer?

What would that mean? A writeTo on one side is the same as a readFrom on the other. 
 
The reasoning is I want to copy an io.Reader where I know the length (so io.CopyN) into a []byte. Thoughts?

 Just io.Read(p []byte) - it reads up to len(p) bytes into p: http://golang.org/pkg/io/#Reader

Mitchell Hashimoto

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Aug 17, 2013, 6:54:35 PM8/17/13
to Benjamin Measures, golang-nuts
Benjamin,

On Sat, Aug 17, 2013 at 3:49 PM, Benjamin Measures
<saint....@gmail.com> wrote:
> On Saturday, 17 August 2013 19:03:28 UTC+1, Mitchell Hashimoto wrote:
>>
>> There is a bytes.Reader for turning a raw []byte into an io.Reader. Why is
>> there no equivalent for io.Writer?
>
>
> What would that mean? A writeTo on one side is the same as a readFrom on the
> other.

It means I can use a []byte as an io.Writer. Having a ReadFrom/WriteTo
implements other interfaces which simply don't satisfy this.

I realize it wouldn't be difficult by any means. It is an easy handful
of lines to write to implement it for []byte. My question was mostly
why doesn't the pair exist in the standard library or if there was a
better way to do what I'm doing.

>
>>
>> The reasoning is I want to copy an io.Reader where I know the length (so
>> io.CopyN) into a []byte. Thoughts?
>
>
> Just io.Read(p []byte) - it reads up to len(p) bytes into p:
> http://golang.org/pkg/io/#Reader

It reads UP TO len(p). If it reads a part and it would start blocking
it might return what it has "so far". I would still have to write a
helper function to copy it all. What I want is a "read until EOF"
(which is io.Copy), so I want an io.Writer implementation so I can use
it on the left side of io.Copy.

For now I've just used "bytes.NewBuffer" since I know exactly how many
bytes will go into there.

Best,
Mitchell

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Benjamin Measures

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Aug 17, 2013, 8:22:37 PM8/17/13
to golan...@googlegroups.com, Benjamin Measures
On Saturday, 17 August 2013 23:54:35 UTC+1, Mitchell Hashimoto wrote:
On Sat, Aug 17, 2013 at 3:49 PM, Benjamin Measures
<saint....@gmail.com> wrote:
> On Saturday, 17 August 2013 19:03:28 UTC+1, Mitchell Hashimoto wrote:
>>
>> There is a bytes.Reader for turning a raw []byte into an io.Reader. Why is
>> there no equivalent for io.Writer?
>
> What would that mean? A writeTo on one side is the same as a readFrom on the
> other.

It means I can use a []byte as an io.Writer. Having a ReadFrom/WriteTo
implements other interfaces which simply don't satisfy this.

I don't think I meant the writerTo interface literally. I was trying to get at the point that if you can't push, pull.
 
>> The reasoning is I want to copy an io.Reader where I know the length (so
>> io.CopyN) into a []byte. Thoughts?
>
>  Just io.Read(p []byte) - it reads up to len(p) bytes into p:
> http://golang.org/pkg/io/#Reader

It reads UP TO len(p). If it reads a part and it would start blocking
it might return what it has "so far". I would still have to write a
helper function to copy it all.

Tbh, I had neglected that aspect of an io.Reader. However, that sounds like a useful helper function, helpful enough to be included as an io.ReadAtLeast, or even better, io.ReadFull: http://golang.org/pkg/io/#ReadFull

Kevin Gillette

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Aug 18, 2013, 1:14:04 AM8/18/13
to golan...@googlegroups.com
On Saturday, August 17, 2013 4:49:33 PM UTC-6, Benjamin Measures wrote:
 Just io.Read(p []byte) - it reads up to len(p) bytes into p: http://golang.org/pkg/io/#Reader

You probably mean io.ReadFull, as that reads precisely len(buf) bytes or returns a non-nil error if it can't. io.Readers are allowed to (and by convention often do) read less than the space available in the provided buffer. 

Mitchell Hashimoto

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Aug 18, 2013, 11:22:59 AM8/18/13
to Ludwig Valda Vasquez, golang-nuts
On Sat, Aug 17, 2013 at 11:15 AM, Ludwig Valda Vasquez <bre...@gmail.com> wrote:
> Why not just use Read method of io.Reader?
>
> http://play.golang.org/p/8ebDizKbr2

Read the rest of the thread and you'll see your answer has already
been answered.

>
> суббота, 17 августа 2013 г., 22:03:28 UTC+4 пользователь Mitchell Hashimoto
> написал:
>>
>> Hello,
>>
>> There is a bytes.Reader for turning a raw []byte into an io.Reader. Why is
>> there no equivalent for io.Writer? I suppose I could use "bytes.Buffer" but
>> I don't want it to grow.
>>
>> The reasoning is I want to copy an io.Reader where I know the length (so
>> io.CopyN) into a []byte. Thoughts?
>>
>> Best,
>> Mitchell
>
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