R devel - Feb 2023 - Line-terminal \ in character consants -- missing from ?Quotes ?

I'm coming across some code that uses the fact the parser ignores a
line-terminal '\', e.g.

identical("\
", "\n")
# [1] TRUE

x = "abc \
def"
y = "abc \ndef"
identical(x, y)
# [1] TRUE

However:
identical("\\n", "\n")
# [1] FALSE

This appears to be undocumented behavior; the closest thing I see in
?Quotes suggests it should be an error:
> Escaping a character not in the following table is an error.
('\n' is in the table, but my understanding is the 'n' is
what's being
escaped v-a-v the "error", which seems confirmed by the third, FALSE,
example above)

Is this a bug, is the omission from ?Quotes a bug, or is this just
undocumented behavior?

Mike C

On 12/02/2023 12:07 a.m., Michael Chirico via R-devel
wrote:
> I'm coming across some code that uses the fact the parser ignores a
> line-terminal '\', e.g.
> 
> identical("\
> ", "\n")
> # [1] TRUE
> 
> x = "abc \
> def"
> y = "abc \ndef"
> identical(x, y)
> # [1] TRUE
> 
> However:
> identical("\\n", "\n")
> # [1] FALSE
> 
> This appears to be undocumented behavior; the closest thing I see in
> ?Quotes suggests it should be an error:
> 
>> Escaping a character not in the following table is an error.
> 
> ('\n' is in the table, but my understanding is the 'n' is
what's being
> escaped v-a-v the "error", which seems confirmed by the third,
FALSE,
> example above)
> 
> Is this a bug, is the omission from ?Quotes a bug, or is this just
> undocumented behavior?
In your first example, you have a backslash which says to escape the 
next char.  The next char is a newline char.  The result is an escaped 
newline, which apparently is a newline.

The same thing happens in the second example.

The third example is an escaped backslash, i.e. a backslash, followed by 
n.  That's not the same as an escaped n, which gives a newline.

So I think the behaviour might be reasonable.

The thing I'd worry about is whether things are handled properly on 
Windows, where the newline is two characters (CR LF).  It might be that 
the backslash at the end of the line escapes the CR, and you get a \r 
out of it instead of a \n.  But maybe not, the parser knows about CR LF 
and internally converts it to \n, so if that happens early enough, 
things would be fine.

Duncan Murdoch