bufio: reading lines is too cumbersome

[bradfitz]

Reading lines from a file is too cumbersome in Go.

People are often drawn to bufio.Reader.ReadLine because of its name, but it has a weird
signature, returning (line []byte, isPrefix bool, err error), and requires a lot of work.

ReadSlice and ReadString require a delimiter byte, which is almost always the obvious
and unsightly '\n', and also can return both a line and an EOF:
http://play.golang.org/p/LSl2Aketpg

This bug is about designing a nicer API for bufio to do the common task of looping over
lines from an io.Reader.

Things to think about:

-- functional or not, like my earlier https://golang.org/cl/6870052/ proposal
and filepath.Walk.

-- if an iterator: return either a line OR an error, but never both?

-- auto-strip line endings before giving it to the caller?

-- CRLF vs LF

-- treat the final line without line endings the same as a line with a line ending?

-- skip, fail early, or truncate lines that are longer than the bufio Reader size?

-- string or []byte? (copy or not?)

etc.

[robpike]

Comment 1:

Status changed to Started.

[mewmew]

Comment 2:

Personally I would feel that an iterator would be more convenient when reading lines.
The error could be similar to that of io.ReadFull:
"The error is EOF only if no bytes were read. If an EOF happens after reading some but
not all the bytes, ReadFull returns ErrUnexpectedEOF. On return, n == len(buf) if and
only if err == nil."
Return ErrUnexpectedEOF (or similar) if the line didn't end with a new line. Only return
EOF when no line was read.
Possibly return an error for truncated lines, ErrLineTruncated (or similar).
Line rog already pointed out: "You almost always want to strip off the final newline"
ref: https://golang.org/cl/6870052/#msg8
If a copy should be made or not is a hard choice. Can't tell at this point what my
thoughts are. It is always easy to provide a wrapper function to make the copy. The
question is, do we want to limit the need of such wrapper functions?

[mewmew]

Comment 3:

When the line is too long (truncated) the error could be ErrPrefix to mimic the behavior
of the isPrefix bool.

[bradfitz]

Comment 4:

Re comment #3: that would be terrible. It shouldn't be possible to accidentally get a
partial line in a convenience API.  If you care about partial lines you can use the
existing bufio.Reader.

[robpike]

Comment 5:

This issue was closed by revision 55ad7b9.

Status changed to Fixed.