Our toy example for this note: We want to generate two random numbers and then output their sum and product. We learned before that random stuff in Haskell needs this scary thing called ´IO´ monad, so we expect this to be difficult.
First we need something to generate random numbers:
Now, let's add and multiply them.
So, what do we get?
*WhyNoIOinput> example1 Sum:9 Product:4 *WhyNoIOinput> example1 Sum:13 Product:21
But wait, if 21 = x * y, then we can never have x + y = 13.
So, what is going wrong here?
The problem is that
makeInt is not evaluated inside example1. And also the
sumAndProductIO function does not run it to get concrete values but is just handing it over to
Imagine I have a calculator with a random number button. Then I can push the button and then tell you the result. Or I can give you the calculator and you push the button yourself. The difference seems unimportant and indeed it is irrelevant if we are only interested in you getting to know one value. But if I also want to give a random number to someone else then it matters: Do I push the button and then give you and your friend the same number or do I give the same calculator, i.e. merely the same way to generate a number to both of you and you push the buttons which can result in different numbers.
We can see what is happening if we use a function that always shows the random numbers it generates:
This gives us:
*WhyNoIOinput> example2 generated a random number: 3 generated a random number: 1 generated a random number: 10 generated a random number: 7 Sum:4 Product:70 *WhyNoIOinput> example2 generated a random number: 4 generated a random number: 9 generated a random number: 10 generated a random number: 2 Sum:13 Product:20
Now it becomes clear that instead of two we always generate four random numbers. Then we get the sum of the first two and the product of the latter two.
Lesson learned: We often do not want ´IO Int´ as an input type.
So, what is the proper way to do it?
First, realise that many of your functions are actually pure!
Use a wrapper function which collects the results of IO functions and then feeds them into the pure function and returns the result.
*WhyNoIOinput> randomSumAndProduct (19,90) *WhyNoIOinput> randomSumAndProduct (12,27)
Final note: Make sure you understand the difference between
x <- makeInt and
let x = makeInt. In the latter variant, x will still be a way to get a value and not a concrete single fixed value!