"Send more money" is a well-known puzzle. Each of the letters D,E,M,N,O,R,S and Y represents a different digit. Moreover, when each letter is mapped to its corresponding digit the equation SEND + MORE = MONEY holds. Since there are 8 letters to be solved, we can simply explore the 10*9*...*3 mappings of letters to digits. This could well be too slow. A little insight can simplify things. Clearly, SEND < 9999 and MORE < 9999. Thus MONEY < 19998 and hence M = 1. Now we have SEND + 1ORE = 1ONEY. Again SEND < 9999 and now 1ORE < 1999 so 1ONEY < 11998. Since M is already bound to 1, O must be bound to 0. A little more thought shows that S must be bound to 8 or 9, and that N = E + 1. Using these insights to reduce the number of solutions that must be explored, write a Prolog predicate soln([D,E,M,N,O,R,S,Y]) that solves this puzzle by binding the correct digits to each of the variables in the list.
Submit your code in a file called money.prolog.