Can we have this convenience as a function in the File module: read(file, start, length)

[Charles Okwuagwu]

Can we have this convenience as a function in the File module?

read(file, start, length)

http://stackoverflow.com/q/37121605/44080

The suggested solution is not immideiately obvious

{:ok, file} = :file.open(source_file, [:read, :binary]) 

:file.position(file, 5) # start_position = 5

:file.read(file, 10) # bytes to read = 10

[eksperimental]

Hi Charles,
thank you for your proposal.

I think it could be an interesting addition. But rather than just a
convenience function, make it behave more like its IO
conterpart: IO.read/2

Where you can pass the byte length, or :line.
Because sometimes is not about the bytes, but about the information
held in a certain number of lines in the file.

I'm thinking something like

File.read(file_path, :line, 1)

# or to read the first five lines
File.read(file_path, :line, 1..5)

I would like to know how the rest of the list feels about it.

[José Valim]

We need to consider that performance wise they are quite different. We can move the cursor when reading lines, we can't move the cursor when reading bytes. We would actually need to load the file into Elixir. It would probably be more interesting to add a File.position instead of hiding the underlying behaviour.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[Charles Okwuagwu]

def read(file, start, length) do
  {ok, f} = :file.open(file, [:binary])
  {ok, data} = :file.pread(f, start, length)
  :file.close(f)
  data
end
We are not reading the whole file, or are we? :file.pread/3

[José Valim]

Not in your example but if you want to do it per line, then yes. 

--

[eksperimental]

I was thinking of delegating stuff to IO.read when dealing with lines

On Thu, 12 May 2016 15:08:15 +0200
José Valim <jose....@plataformatec.com.br> wrote:

> Not in your example but if you want to do it per line, then yes.
>
> On Thursday, May 12, 2016, Charles Okwuagwu <okwua...@gmail.com>
> wrote:
>
> > def read(file, start, length) do
> > {ok, f} = :file.open(file, [:binary])

> > * {ok**, data} = :file.pread(f, start,** length)*
> > :file.close(f)
> > dataend
> >
> > We are not reading the whole file, or are we? *:file.pread/3*

> >
> >
> > On Thursday, May 12, 2016 at 1:36:05 PM UTC+1, José Valim wrote:
> >>
> >> We need to consider that performance wise they are quite
> >> different. We can move the cursor when reading lines, we can't
> >> move the cursor when reading bytes. We would actually need to load
> >> the file into Elixir. It would probably be more interesting to add
> >> a File.position instead of hiding the underlying behaviour.
> >>
> >>

> >> *José Valim*

[Charles Okwuagwu]

I think we are best served with two functions:

read_bytes(file, start, bytes)

read_lines(file, start, lines)

You an then optimize as required for each specific case

[Charles Okwuagwu]

Please is this still being considered?

Thanks

[José Valim]

> It would probably be more interesting to add  a File.position instead of hiding the underlying behaviour. 

My position on the topic is the same. Your solution to the problem is 3 lines of code. It doesn't seem to be a common use case either, so I don't see the reasoning in adding it to Elixir. If the issue is with discovery, I would rather add File.position, as mentioned above, instead of providing convenience functions without adding the primitive operations.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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