lua - Corona Storyboard, declaring an object then making it moveable, nil value attempt -

i creating game, , in level 1 want load few images represent letters, , want add functionalities them. 1 of them ability move them.

so inside enterscene is

function scene:enterscene(event) ...     leta = display.newimage("media/letters/a.png", display.contentwidth/4 - 20, display.contentheight/5 - 18)      letc = display.newimage("media/letters/c.png", display.contentwidth/4 + 35, display.contentheight/5 - 18)     letr= display.newimage("media/letters/r.png", display.contentwidth/4 + 90, display.contentheight/5 - 18)     lete=display.newimage("media/letters/e.png", display.contentwidth/4 + 145, display.contentheight/5 - 18)      screengroup:insert(leta)     screengroup:insert(letc)     screengroup:insert(letr)     screengroup:insert(lete)      leta:addeventlistener("touch", leta)     letc:addeventlistener("touch", letc)     letr:addeventlistener("touch", letr)     lete:addeventlistener("touch", lete) 

now added moving function leta, is

function leta:touch(event)      if event.phase=="began"         display.getcurrentstage():setfocus(self, event.id)         self.isfocus = true          self.markx = self.x         self.marky = self.y          elseif self.isfocus             if event.phase=="moved"                 self.x = event.x - event.xstart + self.markx                 self.y = event.y - event.ystart + self.marky             elseif event.phase=="ended" or event.phase == "cancelled"                 display.getcurrentstage():setfocus(self,nil)                 self.isfocus = false             end         end          return true end 

then when try scene gives me error line of function leta:touch(event), says "attempt index global 'leta'.

what do then? designed because want when user clicks play, load letters, , if presses back, unload them screen.

you can instead, declare touch function via ":" (colon) leta or other objects not tables.

local leta = display.newimage("media/letters/a.png", display.contentwidth/4 - 20, display.contentheight/5 - 18)  leta.touch = function(self,event)      if event.phase=="began"         display.getcurrentstage():setfocus(self, event.id)         self.isfocus = true          self.markx = self.x         self.marky = self.y      elseif self.isfocus         if event.phase=="moved"             self.x = event.x - event.xstart + self.markx             self.y = event.y - event.ystart + self.marky         elseif event.phase=="ended" or event.phase == "cancelled"             display.getcurrentstage():setfocus(self,nil)             self.isfocus = false         end     end      return true end   leta:addeventlistener("touch")