Teach Yourself Scheme: 4.1 'when' and 'unless' (idea)

when and unless are convenient conditionals to use when only one branch (the ``then'' or the ``else'' branch) of the basic conditional is needed.

(when (< (pressure tube) 60)
   (open-valve tube)
   (attach floor-pump tube)
   (depress floor-pump 5)
   (detach floor-pump tube)
   (close-valve tube))

Assuming pressure of tube is less than 60, this conditional will attach floor-pump to tube and depress it 5 times. (attach and depress are some suitable procedures.)

The same program using if would be:

(if (< (pressure tube) 60)
    (begin
      (open-valve tube)
      (attach floor-pump tube)
      (depress floor-pump 5)
      (detach floor-pump tube)
      (close-valve tube)))

Note that when's branch is an implicit begin, whereas if requires an explicit begin if either of its branches has more than one form.

The same behavior can be written using unless as follows:

(unless (>= (pressure tube) 60)
   (open-valve tube)
   (attach floor-pump tube)
   (depress floor-pump 5)
   (detach floor-pump tube)
   (close-valve tube))

Not all Schemes provide when and unless. If your Scheme does not have them, you can define them as macros (see chap 8).

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