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* Add community knowledge on AL boolean operators not short-circuiting AL gives no short-circuit (lazy) evaluation guarantee for and/or/xor — neither the AL operators nor the boolean operators documentation defines a lazy evaluation order. LLMs trained on C#, JavaScript, or SQL assume the left operand guards the right, which produces conditions where a guard does not protect an unsafe subscript or a field read after a failed Get, and where an expensive operand is paid on every path. Adds community/knowledge/performance/boolean-operators-do-not-short-circuit.md with good/bad AL companions. The guidance prefers nested if when one operand depends on another, while keeping and/or legitimate for operands that are independently safe and cheap, so a reviewer does not flag harmless bound checks. This is the first article in a community performance domain; the Microsoft performance review leaf skill already sources candidates by domain across every enabled layer, so no skill change is needed to reach it. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> * Add case true of pattern for long condition chains Follow-up from PR review: nested if is the right answer for two or three dependent conditions, but past that the nesting becomes the problem. AL's case statement is the flat alternative — the control statements documentation states a value set "must be an expression or a range" and that the first matching value set executes, so case true of / case false of accept boolean expressions and stop at the first match. That is the laziness the boolean operators do not provide. Adds case-true-of-for-long-condition-chains.md with good/bad AL companions: case false of for guard chains where every condition must hold, case true of for first-match dispatch. The bad sample shows both failure shapes — a five-level if ladder, and the worse escape of collapsing it into an and chain, which trades nesting for a real defect. Cross-links both articles, and adds the threshold to the short-circuit article's Best Practice so following it does not lead to a deep ladder. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> * Comma-group the pure value sets in the case false of sample Review feedback: the five value sets sharing exit(false) should be comma separated. Applied to the three that are pure field reads with no order dependency. The Get and the Blocked read keep their own value sets. The documentation guarantees that the first matching value set executes, which orders matching across separate value sets; it says nothing about evaluation within one comma-separated set, and the natural lowering of that is an equality-or chain — where AL's or does not short-circuit. Grouping the Get with the checks that must precede it would rest the sample's correctness on undocumented behaviour, which is the defect these two articles exist to prevent. Encodes the boundary in the Best Practice section so the grouping is applied where it is safe and not where it is not, and scopes the repeated-exit detection signal explicitly to nested chains so the case sample does not read as its own anti-pattern. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> * Use a single exit(false) for the whole case false of chain Review decision: all five conditions share one action, so they share one comma-separated value set with a single exit(false). Aligns the article's Best Practice with the sample — it previously told authors to keep side-effecting conditions in their own value set, which the sample no longer does — and drops the now-contradictory wording about listing the failure action per condition. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> * Drop the parentheses from the case false of value sets Applies NKarolak's review suggestion: a case value set needs no parentheses around a comparison. Encodes the rationale in the Best Practice section, since it is a real advantage of the pattern and is not documented elsewhere in the repo. The AL operator hierarchy places and/xor above the comparison operators and or just above them too, so parentheses are mandatory in an and chain — A = B and C = D misparses without them — while a case value set has no and to bind tighter and needs none. That inverts the precedence most developers arrive with from C#. Also notes in the bad sample that its parentheses are not optional, so the two samples contrast on parentheses as well as on evaluation. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> * Address correctness review: or/xor pattern, case value-set ordering Fixes two points from JesperSchulz's review on PR #136. 1. boolean-operators-do-not-short-circuit.md gave one fix — nested if — for and, or, and xor alike. That's only correct for and: nesting if A then if B then Action drops the A-true/B-false case of A or B, silently changing the result. Gives or its own early-exit pattern (if A then exit(true); exit(B)), warns explicitly against applying the and-rewrite to or, and clarifies that xor is not a short-circuit candidate in any language since its result always depends on both operands. Adds an or fixture (IsEligibleForFreeShipping) to both samples so an agent has a concrete pattern to match instead of extrapolating from the and-only examples. 2. case-true-of-for-long-condition-chains.md derived stop-at-first-match for one comma-separated value set from the documentation's guarantee about the first matching value set — plural, i.e. ordering across value sets, which is not the same claim. The good sample's Item.Get / Blocked pair depended on the one the docs don't make. Restructures the sample to only comma-group the three pure, order-independent checks; Get and Blocked keep their own value sets, in order, relying solely on the guarantee that is actually documented. Description and Anti Pattern now state that boundary so it isn't re-collapsed later. Also carries forward a parenthesis fix (not Item.Blocked in the collapsed bad sample) that was made two commits ago but never landed. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> --------- Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
36 lines
4.4 KiB
Markdown
36 lines
4.4 KiB
Markdown
---
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bc-version: [all]
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domain: performance
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keywords: [short-circuit, lazy-evaluation, boolean-operators, nested-if, guard, and-operator, or-operator, xor-operator, early-exit]
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technologies: [al]
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countries: [w1]
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application-area: [all]
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---
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# AL boolean operators do not short-circuit
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## Description
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AL gives no short-circuit (lazy) evaluation guarantee for `and`, `or`, and `xor`: every operand of a boolean expression is evaluated, even when the leftmost operand already determines the result. Neither the AL operators documentation nor the boolean operators documentation defines a lazy evaluation order, so code must not depend on one. Developers arriving from C#, JavaScript, or SQL routinely assume the left operand guards the right; in AL it does not. `xor` is not actually a short-circuit candidate in any language — its result depends on both operands regardless of their values, so there is nothing to skip — but AL still evaluates both operands unconditionally, so neither should carry a cost or a risk the developer assumed the other would guard against. For `and` and `or`, the right operand still runs even when the left already decides the result, so its cost is paid on every evaluation, and a check intended to protect an unsafe expression — an array subscript, a division, a field read that is only valid after a successful `Get` — does not protect it.
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## Best Practice
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For an `and`-shaped guard — a condition that must hold before the next operand is safe or worth evaluating — split into nested `if` statements: the guarding or cheapest condition in the outer `if`, the dependent or expensive one in the inner `if`. This preserves the result, since `if A then if B then Action` matches `if A and B then Action` exactly. Where there is no `else` branch, nesting is a pure win; where there is one, extract the conditions into a helper procedure that exits early instead.
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For an `or`-shaped condition, do not nest: nesting `if A then if B then Action` drops the case where `A` is true and `B` is false, silently changing the result of `A or B`. Exit as soon as the cheap or safe operand already decides the outcome, and reach the other operand only on the path where it can still change the result — `if A then exit(true); exit(B);` for a boolean return, or `if A then Action else if B then Action;` when both branches share one action.
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`xor` has no equivalent rewrite, because its result always depends on both operands; the only actionable guidance is to keep both operands of an `xor` cheap and free of side effects, since AL evaluates both unconditionally.
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Where a chain of `and`-guards runs past about three conditions, stop nesting and use a `case` statement instead — see `case-true-of-for-long-condition-chains.md`. Keep `and` and `or` for operands that are independently safe and cheap — in-memory field comparisons, enum tests, bound checks — where combining them reads better and costs nothing.
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See sample: `boolean-operators-do-not-short-circuit.good.al`.
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## Anti Pattern
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A single condition that joins a guard with an operand depending on that guard, or with an expensive operand, using `and` or `or`. The consequence is either wasted work on every evaluation — a database call or validation procedure invoked even when the outcome is already decided — or a runtime error or silently wrong result that the guard was written to prevent. Applying the `and` fix to an `or` condition is a distinct mistake: rewriting `A or B` as nested `if`s drops the `A`-true/`B`-false case instead of preserving it. Detection signals: an operand that indexes an array or list with a variable whose bounds are checked in a sibling operand; `Record.Get(...)` or a `Find`/`IsEmpty` call as one operand of `and` with a field read of the same record as another; an expensive or unsafe operand combined with `or` next to a condition that alone already makes the result true; a boolean-returning procedure call combined with a cheap field test. The pattern is common in code ported from a language that does short-circuit, and in conditions grown by appending a clause to an existing `if`.
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See sample: `boolean-operators-do-not-short-circuit.bad.al`.
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## See also
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`case-true-of-for-long-condition-chains.md` covers what to do when nesting an `and`-guard chain would go more than about three levels deep. `microsoft/knowledge/performance/apply-guards-before-get.md` covers the related ordering rule for statements rather than operands.
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