Tip #650: abbreviation that prompts whether to expand it or not

You can define abbreviation in such a way that it will ask whether to expand it or not. The trick is to define it as insert-mode mapping with special body, not as abbreviation.
Here is how to define it:

   function! MyExpand(abbr,expansion)
     let answer=confirm("Expand '".a:abbr."' [y] ", "&Yes;\n&No;")
     if answer==2
        exec "normal! a".a:abbr
     else
         exec "normal! a".a:expansion
     endif
   endfunction

imap ABC <esc>:call AskExpand("ABC","...expansion for ABC ...")<cr>a
imap XYZ <esc>:call AskExpand("XYZ","...expansion for XYZ ...")<cr>a

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Additional Notes