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when

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.
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