# A With expression local variable specification without head List

I just noticed a new form of With in its documentation:

With[$x=x_0$, $expr$]

specifies that all occurrences of the symbol $x$ in $expr$ should be replaced by $x_0$.

Note the lack of braces around first argument. I can see this also in documentation center of v10.2.

But when I try to use it, I get the standard error message, as in previous versions.

\$Version
With[x = y, x]
(* "10.2.0 for Linux x86 (64-bit) (July 6, 2015)" *)


With::lvlist: Local variable specification x=y is not a List. >>

(* With[x = y, x] *)


Is this another symptom of documentation driven development?

Reported to WRI as [CASE:3390526]

• @MarcoB on Win7 it appeared in 10.1->10.2 step. – Kuba Jul 23 '15 at 9:49
• It is a curious choice to implement this syntax now. It would have made good sense to do so initially (perhaps along with {a, b, c, ...} = {a0, b0, c0, ...}) for the sake of consistency with other functions, but to leave it until after version 10 seems to be to introduce a backward-compatibility constraint to no real benefit. – Oleksandr R. Jul 23 '15 at 10:34
• That bit of documentation is a bug so feel free to add that tag. One thing this was not is documentation driven development. – Daniel Lichtblau Jul 23 '15 at 13:36
• @Mr.Wizard It was implemented in the development versions of 10.2 for a while, but due to several issues it caused, as well as long internal discussions, it was shelved for the time being. – Stefan R Jul 23 '15 at 18:24
• It's not likely to come back.As noted by @StefanR. it caused actual trouble (conflict with existing evaluation semantics). – Daniel Lichtblau Jul 23 '15 at 20:33