when

Introduction

In Scheme, while if is elegant and versatile, it can become confusing when used without an explicit else. This is particularly true when the intention is to execute a single branch of code only when a condition is true, with no alternative action for the false case. In such scenarios, the when construct provides a clearer and more concise alternative.

The basic form of when looks like this:

(when test-is-true
  do-this
  do-that)

• If the test evaluates to true (#t), all the expressions in the body of the when construct are executed sequentially.
• If the test evaluates to false (#f), nothing happens, and no values are returned.

Example

(when (< 0 1)
  (gimp-message "Condition is true!")
  (gimp-message "Executing additional actions."))

Contrasting if and when

To better understand the difference between if and when, consider the following example where both are used together:

(if (= 0 1)
  (gimp-message "This will not run")
  (when (< 0 1)
    (gimp-message "The 'when' condition is true!")
    (gimp-message "Executing multiple actions within 'when'.")))

Explanation:

1. The if Condition:

   • The test (= 0 1) checks if 0 is equal to 1.
   • Since this is false (#f), the else branch of the if is executed.

2. The when Construct in the else Branch:

   • The when test (< 0 1) checks if 0 is less than 1.
   • Since this is true (#t), all the expressions within the body of the when are executed sequentially:
     • First, it prints "The 'when' condition is true!".
     • Then, it prints "Executing multiple actions within 'when'.".

Why Use when Here?

• Using when instead of another if simplifies the logic when there is no need for an explicit else branch for the condition.
• when makes it clear that only the true branch is relevant, reducing potential confusion.

Summary

• Use if when you need both a true and false branch.
• Use when when there is only a single branch for the true case, especially when multiple actions need to be executed.
• Combining if and when can help structure more complex conditionals clearly and concisely.