Map comma in select mode to exit select mode, enter insert mode, append a comma

Question

Is it possible to remap comma in select mode to exit select mode, enter insert mode, [optionally: go the end of the last selection (which the cursor seems to be for ultisnips)] and append a comma? This can be very useful while using a ultisnips snippet below to quickly add new items to a opt=val type argument list: new ,opt1=val1,opt2=val2,opt3=val3

snippet , "add one more item" i
,${1:option}=${2:value}
endsnippet

I tried something like this:

smap , <Esc><Esc>a,

but it doesn't do that, and ends up deleting the last selection.

Answer 1

You must use snoremap as your mappings' RHS contains LHS. And this is a good habit anyway.

exit select mode, enter insert mode, [optionally: go the end of the last selection (which the cursor seems to be for ultisnips)] and append a comma?

For simplicity, let's assume that :h 'selection' is inclusive. Then we have

snoremap , <Esc>g`>a,