write(to::IO, from::IO)

[samoconnor]

I have some code like this:

    stream = s3(aws, "GET", key)
    open(filename, "w") do file
        while !eof(stream)
            write(file, readavailable(stream))
        end
    end

With write(to::IO, from::IO) it could be reduced to:

stream = s3(aws, "GET", key)
open(filename, "w") do f write(f, stream) end

or even just...

write(filename, s3(aws, "GET", key))

This is not just about terse code.
The write(readavailable()) loop is potentially quite memory inefficient. If the task has been suspended for a while, the stream could have many MB available and depending on how stream buffering is implemented large buffers could be duplicated.

It seems like it would be better to have a write(to::IO, from::IO) API so that shortcut implementations can be created to directly connect two streams at a low level where that is more efficient.
e.g. this could map to sendfile() in some cases.

The default can just be:

write(to::IO, from::IO) = while !eof(from) write(to, readavailable(from)) end

[vtjnash]

I agree

[StefanKarpinski]

+1

[samoconnor]

Included in #14660
closing

[samoconnor]

See https://github.com/JuliaLang/julia/blob/master/base/io.jl#L175

[swt30]

Ah, thank you - I was thinking about this just last night while writing from one stream to another. It seemed odd to me that this method didn't already exist.