Question

Can anyone help me out understanding the various parameter passing modes in Scheme? I know Scheme implements parameter passing by value. But how about other modes?

Is there any good documentation for parameter passing in Scheme?

Answer 1

Scheme has only call-by-value function calls. There are other alternatives that can be implemented within the language, but if you’re a beginner then it’s best to not even try them at this point. If you’re looking for a way to pass values “by reference” — then one option that can sort of make it is to use macros, but you really shouldn’t go there. Instead, some Scheme implementations like PLT Scheme provide a “box value”: this is a kind of a container that is used like this:

• You create a box holding <something> with (box <something>)
• You get the value that is stored in a box with (unbox <some-box>)
• You change the value that is stored in a box with (set-box! <some-box> <new-value>)

Given these two, you can use such box objects “by value”, but their contents is actually a reference. This is very much like C, where all values (most, actually) are passed by-value, yet some of these values can be pointers that you can mutate. BTW, it’s best to avoid even these: in Scheme, functional programming is the more common choice and it is therefore better to start with that.

(Once you are more fluent with Scheme, and if you’re using a Scheme with sufficient abstractions, then you can learn how to mimic lots of alternatives too.)