what is the diffirent between 2 part codes

--- Quote Start ---

as far as transition is concerned the first two transitions should be equivalent e.g.:

when idel =>      
   if data_in='1' then       
         next_state<=d1;       
  --else  -- not needed       
      --next_state<=idel;      
  end if;
when d1 =>      
    if data_in='0' then        
         next_state<=d10;        
   -- else        
   -- next_state<=d1;       
   end if;

however i have my doubts since you have a default statement at top:

next_state <= current_state;

--- Quote End ---

i can understand like that:

when idel =>      
   if data_in='1' then       
         next_state<=d1;            
  end if;

in this code, if data_in is 1 then next_state will be d1, but when data_in is 0 then we dont know next_state will be

when idel =>      
   if data_in='1' then       
         next_state<=d1;       
         else   
         next_state<=idel;      
  end if;

in this code, if data_in is 1 then next_state will be d1, but when data_in is 0 then next_state will be idel, so is this the different ???