Chapter 6: Control Structures

Objectives

By the end of this chapter you will be able to:

• Use if, case, and loops appropriately in AL
• Choose between for, while, and repeat..until based on intent
• Apply safe error handling patterns for business rules
• Write control flow that scales to real data volumes

6.1 Conditional statements (if and case)

Use if for binary choices or short decision trees, and case when there are several discrete branches.

if Score >= 90 then
    Grade := 'A'
else
    if Score >= 80 then
        Grade := 'B'
    else
        Grade := 'C';

case DocumentStatus of
    DocumentStatus::Open:
        HandleOpen();
    DocumentStatus::Released:
        HandleReleased();
else
    Error('Unsupported status: %1', Format(DocumentStatus));
end;

6.2 Looping constructs

Choose the loop type that matches the business behavior:

• for: known iteration count (for example, fixed installments)
• while: condition-driven iteration
• repeat..until: record traversal after FindSet

if Customer.FindSet() then
    repeat
        ProcessCustomer(Customer);
    until Customer.Next() = 0;

Chapters 9 and 13 use this pattern heavily.

6.3 Error handling and assertions

Use Error to stop invalid transactions early.

if Amount <= 0 then
    Error('Amount must be greater than zero.');

In tests (Chapter 14), prove behavior with assertions instead of checking it by hand in the UI.

6.4 Control-flow quality guidelines

• Keep nesting shallow by extracting helper procedures.
• Fail fast on invalid data.
• Never use a loop when a filtered set would do (see Chapter 13).

6.5 Control-structure decision map (visual guide)

flowchart TD
    A[Need branching or iteration?] --> B{Single yes/no decision?}
    B -->|Yes| C[Use if]
    B -->|No| D{Multiple discrete states?}
    D -->|Yes| E[Use case]
    D -->|No| F{Known number of iterations?}
    F -->|Yes| G[Use for]
    F -->|No| H{Iterating record set after FindSet?}
    H -->|Yes| I[Use repeat..until]
    H -->|No| J[Use while]

Chapter summary

In this chapter you:

• Applied AL conditionals and loops, with clear criteria for choosing each one
• Used repeatable record-iteration patterns
• Added safe, explicit error handling to business rules
• Prepared control-flow techniques that codeunits and tests reuse

Tasks

1. Branching choice. Replace one overgrown if chain with a case where appropriate.
2. Decision fallback. Add an else branch (or case else) that raises an Error for unsupported state values.
3. Loop correction. Refactor one loop to use FindSet + repeat..until for record traversal.
4. Loop intent check. Convert one loop to for when the iteration count is known ahead of time.
5. Guard clause. Add one fail-fast Error that protects invalid input before any data changes.
6. Try it yourself. Extract one nested block into a helper procedure to reduce nesting depth.

Check your work: compare your answers with the Chapter 6 solutions, then verify your loop and branching style aligns with Chapters 9, 13, and 14 patterns.