Imagine I have a function with a domain of all integers bigger than 0. I want the result of other inputs to be undefined. For the sake of simplicity, let's say this is the increment function. In Haskell, I could achieve this with something like
f :: Integer -> Integer
f x
| x > 0 = x + 1
| otherwise = undefined
Of course, the example is quite gimped but it should be clear what I want to achieve. I'm not sure how to achieve the similar in Scheme.
(define (f x)
(if (> x 0)
(+ x 1)
(?????)))
My idea is to just stick an error in there but is there any way to replicate the Haskell behaviour more closely?
Your question is related to this one which has answers pointing out that in R5RS (which I guess MIT scheme partially supports?), the if with one branch returns an "unspecified value". So the equivalent to the haskell code should be:
(define (f x)
(if (> x 0)
(+ x 1)))
You probably already know this: in haskell undefined is defined in terms of error, and is primarily used in development as a placeholder to be removed later. The proper way to define your haskell function would be to give it a type like: Integer -> Maybe Integer.