[Jprogramming] J-like approach for merging numbers in a list

[Ric Sherlock]

I'm looking for a J-like approach to a verb for merging adjacent numbers in
a row where the numbers are the same. For example

mergerow 4 4 4 4

8 8

mergerow 8 8

16

mergerow 16

16

mergerow 4 4 4

8 4

mergerow 8 4

8 4

mergerow 4 8 8 4

4 16 4


My version below which I've used as part of the answer to a RosettaCode task
<http://rosettacode.org/wiki/2048#J> is very non J-like:

mergerow=: verb define
  len=. <:@# y
  row=. i. i=. 0
  while. (i <: len) *. len >: 0 do.
    if. =/ (i , i+1) { y,_9 do.
      row=. row, +: i { y
      i=. i + 2
    else.
      row=. row, i { y
      i=. i + 1
    end.
  end.
  row
)

I thought I had a good solution with:

mergerow=: rplc (2&# ; +:)&>@~.

But this seemed to cause unpredictable behaviour when used as part of the
whole solution.