Any fully-compliant R5RS really. I'm talking about the concepts of exactness and the numerical tower in particular. The notion that an integer is a subset of a real number instead of a whole separate type is conceptually very nice.
Yes, and: "Implementations of Scheme are not required to implement the whole tower of subtypes given in section 6.2.1, but they must implement a coherent subset consistent with both the purposes of the implementation and the spirit of the Scheme language. For example, an implementation in which all numbers are real may still be quite useful." (From your URL)
The suggestion in the R5RS spec is that you shouldn't need to worry about the segregation of integer and real types (unless you need to). And indeed, why should you?
It's a conceptual shift away from char, int, long, float and double, which exist only because they're easier for the computer to deal with. Scheme was the first language I used that attempted to treat numbers like numbers.
Besides, Exactness is required in R5RS and the full numerical stack is no longer optional in R6RS.
http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z...