gistfile1.txt

IMPLEMENTATION Rabbits

IMPORT  Denotation  ONLY ++
        Nat         ONLY nat ! 0 1 2 - + > =
        NatConv     ONLY `
        String      ONLY string
        StringConv  ONLY `
        Com         ONLY ans:SORT
        ComCompose  COMPLETELY
        Stream      ONLY input stdIn readLine
                         output stdOut writeLine
                         write : output ** denotation -> com[void]

-- FUN main : com[void]
DEF main ==
    write(stdOut, "For which generation do you want to know the number of rabbits? ")
        & (readLine(stdIn) & (\\ in. processInput(in`)))

FUN processInput : denotation -> com[void]
DEF processInput(ans) ==
    LET generation == !(ans)
        bunnies    == rabbits(generation)
        result     == "In the "
                      ++ (generation`)
                      ++ ". generation there are "
                      ++ (bunnies`)
                      ++ " couples of rabbits."
    IN writeLine(stdOut, result)

FUN rabbits : nat -> nat
DEF rabbits(generation) ==
    IF generation = 0 THEN 1
    IF generation = 1 THEN 1
    IF generation > 1 THEN rabbits(generation - 1) + rabbits(generation - 2) FI