ioSystem

want IO system to be universal regardless of which operations we choose to use
want IO system to support operations on whatever target platform we're compiling to (eg no embedding assembly code in utlc code)
want IO system to be based on IO values rather than side effects that happen while we're computing
want IO system to be as simple as possible

IO a b = a, (b -> IO a b)
choosing a to be maybe string and b to be maybe string, we can make string IO where the program chooses when it wants to input and when it wants to output
we can combine different IO effects by using eithers (tagged unions)
we can add forks by making one of the options for a be IO a b
we can add an option to terminate the program by making a be a maybe type, where none indicates that we should terminate and anything else is normal IO
we can allow the program return what input it wants to take (if any) by having a be (action, desiredInput)

example of repeated print/input IO where a = b = string:
main = "" , mainLoop
mainLoop = inp -> inp , mainLoop
this IO repeatedly reads input and then echoes it back

we can have the compiler feed IO options into main, if we want to have the type of a be flexible and not known when writing the program:
main = ioFuncs -> mainLoop ioFuncs "x"
mainLoop = ioFuncs -> toPrint -> (ioFuncs "print" toPrint) , (inp -> mainLoop ioFuncs toPrint)
(this IO prints "x" and then repeatedly echoes back its input)
when we do it this way, it doesn't matter what exact IO functions we have available, as long as one of them is "print"

another example program (a countdown that doesn't take input):
main = ioFuncs -> mainLoop 10
mainLoop = ioFuncs -> n -> if (n == 0) (ioFuncs "terminate" , _) else ((ioFuncs "print" n , ioFuncs "noInputWanted") , (_ -> mainLoop ioFuncs (n-1)))