Why doesn't gJip turn a paragraph into a line?

Question

Is there a good reason why gJip doesn't turn a paragraph into a line?

If not, how could I make it work that way?

Answer 1

The answer is simple: gJip doesn't work, because gJ is not an operator. See :h operator for which operators exist. The easy workaround is to use visual mode, and then join all the lines, e.g. use vipgJ, because in visual mode gJ will join all those selected lines (see :h v_gJ).

If you want to know the reason, why gJ and also J has not been implemented as an operator, you have to ask Bill Joy (for J in original vi) and Bram Moolenaar (for gJ only available in Vim). My guess is, that using e.g. a motion for the join operation does not always make sense, e.g. the command Jw is non-sense.