foo.vim

function! s:dump_api()
  let api = msgpackparse(systemlist('nvim --api-info'))[0]

  for v in api.functions
    echohl Function
    echomsg v.name._VAL[0]
    echohl NONE

    echohl Title
    echon '('. join(map(v.parameters, 'v:val[1]._VAL[0] .":". v:val[0]._VAL[0]'), ', ') .')'
    echohl NONE

    echon ' -> '

    echohl Constant
    echon v.return_type._VAL[0]
    echohl NONE

    echohl Comment
    echon ' async: '. (v.async ? '✓' : '☓') .', can fail: '. (has_key(v, 'can_fail') && v.can_fail ? '✓' : '☓')
    echohl NONE
  endfor
endfunction

command! DumpAPI call s:dump_api()