Parameter Passing in Scheme

Question

Hi All,

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.)

Answer 2

To add a bit more...

The four fundamental parameter-passing conventions are call-by-value, call-by-reference, call-by-name, and call-by-need. Scheme, as a "mostly functional" language, relies on call-by-value; variables, once created, generally aren't changed. The other three conventions are pretty similar and you can still do them in Scheme by passing around your values in boxes (using box and unbox), and the boxes act as pointers to values.

Generally, if you find that you need to use call-by-reference in a function, you should probably rethink how you're implementing the function and make it purely functional. Modifying a variable after it's been created using set! is a "side-effect" and is typically avoided in functional programming.