Can you return nothing from a function in Scheme?

Question

I'm writing a scheme interpreter, and in the case of an if statement such as:

(if (< 1 0) 'true)

Any interpreter I've tried just returns a new prompt. But when I coded this, I had an if for whether there was an alternative expression. What can I return in the if such that nothing gets printed?

(if (has-alternative if-expr)
  (eval (alternative if-expr))
  #f) ;; what do I return here?

Solution 1

According to the R6RS specification:

If <test> yields #f and no <alternate> is specified, then the result of the expression is unspecified.

So go wild, return anything you want! Although #f or '() are what I, personally, would expect.

Solution 2

Scheme can indeed return no values:

   > (values)

In R5RS the one-armed form of if is specified to return an unspecified value. That means it is up to you, to decide which value to return. Quite a few Schemes have chosen to introduce a specific value called "the unspecified value" and returns that value. Others return "the invisible value" #<void> and the REPL is written such that it doesn't print it.

   > (void)

At first one might think, this is the same as (values), but note the difference:

  > (length (list (void)))
  1

  > (length (list (values)))
  error>  context expected 1 value, received 0 values
  (Here (list ...) expected 1 value, but received nothing)

If #<void> is part of a list, it is printed:

  > (list (void))
  (#<void>)

Solution 3

A number of Schemes (PLT, Ikarus, Chicken) have a void type, which you can produce with (void).

In PLT at least, void is what you get when you do (when (< 1 0) #t).

(PLT v4 doesn't allow if without an else clause.)

Solution 4

When the return value is unspecified you can return what you like; the user just can't rely on that value being there, ever, or across implementations.

Solution 5

Weird people would return 'nil or '|| (the empty symbol). The problem is to return a symbol that cannot be return by (eval (alternative if-expr)) to avoid confusion.

If anything can be returned by (eval (alternative if-expr)) and you still want to know whether you came in this alternative or not, you have to pack the result with more information :

(if (has-alternative if-expr)
  (cons #t (eval (alternative if-expr)))
  (cons #f #f))

Thus, the result is a cons cell. If its car is #t, then you evaled something. If it is #f, you didn't.

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Comments

[Andrey Fedorov]

So this just defaults to whatever the scheme interpreter's language does? Nothing wrong, unless you're trying to implement R6RS compatible Scheme on top of a non-R6RS compatible Scheme :-P

[Matt Ellis]

The same is true in R5RS as well.

[Jyaan]

I prefer #f since (if '() ...) will give the #t branch (have to test with null?).