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Parsing expressions

The code can be improved again with recursion:

EXP = NUM ('+' EXP | epsilon)

let rec exp = 
  num'       && fun n1 -> 
  ((sym '+') && fun _  -> 
   exp       && fun n2 -> 
   ret (n1+n2)) 
  || (ret n1);;

exp "12+34+56";;

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