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Address correctness review: or/xor pattern, case value-set ordering

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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>
This commit is contained in:
António Silva 2026-09-02 11:24:09 +01:00
parent 0d5150c47b
commit 47000475e3
6 changed files with 54 additions and 14 deletions
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13
community/knowledge/performance/boolean-operators-do-not-short-circuit.bad.al
View file

@ -15,4 +15,17 @@ codeunit 50541 "Perf Sample NoShortCircuit Bad"
// Get failed, so the result is taken from a record that was never loaded.
exit((CustomerNo <> '') and Customer.Get(CustomerNo) and (Customer.Blocked <> Customer.Blocked::" "));
end;
procedure IsEligibleForFreeShipping(SalesHeader: Record "Sales Header"): Boolean
begin
// HasActiveLoyaltyBenefit runs even when the amount alone already qualifies,
// paying for the costly check on every evaluation instead of only the path
// where it can still change the outcome.
exit((SalesHeader."Amount Including VAT" >= 1000) or HasActiveLoyaltyBenefit(SalesHeader."Sell-to Customer No."));
end;
local procedure HasActiveLoyaltyBenefit(CustomerNo: Code[20]): Boolean
begin
exit(CustomerNo <> '');
end;
}

17
community/knowledge/performance/boolean-operators-do-not-short-circuit.good.al
View file

@ -21,4 +21,21 @@ codeunit 50540 "Perf Sample NoShortCircuit Good"
exit(false);
exit(Customer.Blocked <> Customer.Blocked::" ");
end;
procedure IsEligibleForFreeShipping(SalesHeader: Record "Sales Header"): Boolean
begin
// 'or' is unsafe here: nesting would also be wrong, since it would drop the
// case where the amount alone already qualifies. Exit as soon as the cheap
// condition already decides the result; the costly lookup runs only on the
// path where it can still change the outcome.
if SalesHeader."Amount Including VAT" >= 1000 then
exit(true);
exit(HasActiveLoyaltyBenefit(SalesHeader."Sell-to Customer No."));
end;
local procedure HasActiveLoyaltyBenefit(CustomerNo: Code[20]): Boolean
begin
// Stands in for a costly check — a webservice call or a large table scan.
exit(CustomerNo <> '');
end;
}

16
community/knowledge/performance/boolean-operators-do-not-short-circuit.md
View file

@ -1,7 +1,7 @@
---
bc-version: [all]
domain: performance
keywords: [short-circuit, lazy-evaluation, boolean-operators, nested-if, guard, and-operator, or-operator]
keywords: [short-circuit, lazy-evaluation, boolean-operators, nested-if, guard, and-operator, or-operator, xor-operator, early-exit]
technologies: [al]
countries: [w1]
application-area: [all]
@ -11,20 +11,26 @@ application-area: [all]
## Description
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. The right operand still runs, 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.
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.
## Best Practice
Split a condition into nested `if` statements whenever one operand is only safe or only worth evaluating once another has passed. The guarding or cheapest condition goes in the outer `if`, the dependent or expensive one in the inner `if`, which makes the dependency explicit and keeps the expensive operand off the path that would have been skipped. 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. Where the chain 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.
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.
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.
`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.
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.
See sample: `boolean-operators-do-not-short-circuit.good.al`.
## Anti Pattern
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. 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; 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`.
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`.
See sample: `boolean-operators-do-not-short-circuit.bad.al`.
## See also
`case-true-of-for-long-condition-chains.md` covers what to do when nesting the conditions 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.
`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.

2
community/knowledge/performance/case-true-of-for-long-condition-chains.bad.al
View file

@ -24,6 +24,6 @@ codeunit 50543 "Perf Sample CaseChain Bad"
// fields are read even when the Get failed. The parentheses are not
// optional either — 'and' binds tighter than '=' and '<>' in AL.
exit((SalesLine.Type = SalesLine.Type::Item) and (SalesLine."No." <> '') and
(SalesLine."Qty. to Ship" > 0) and Item.Get(SalesLine."No.") and (not Item.Blocked));
(SalesLine."Qty. to Ship" > 0) and Item.Get(SalesLine."No.") and not Item.Blocked);
end;
}

14
community/knowledge/performance/case-true-of-for-long-condition-chains.good.al
View file

@ -4,14 +4,18 @@ codeunit 50542 "Perf Sample CaseChain Good"
var
Item: Record Item;
begin
// 'case false of' matches the values in order and stops at the first match,
// so each condition is reached only when the previous one passed — and
// Item.Get is never called for a non-item line.
// 'case false of' matches value sets in order and stops at the first match.
// The first three checks are pure and order-independent, so they share one
// value set. Get and Blocked are each their own value set, in order, because
// the ordering the documentation guarantees is across value sets, not within
// one — Item.Get must run, and succeed, before Blocked is read.
case false of
SalesLine.Type = SalesLine.Type::Item,
SalesLine."No." <> '',
SalesLine."Qty. to Ship" > 0,
Item.Get(SalesLine."No."),
SalesLine."Qty. to Ship" > 0:
exit(false);
Item.Get(SalesLine."No."):
exit(false);
not Item.Blocked:
exit(false);
end;

6
community/knowledge/performance/case-true-of-for-long-condition-chains.md
View file

@ -11,17 +11,17 @@ application-area: [all]
## Description
Because AL gives no short-circuit guarantee for `and` and `or`, a chain of conditions that must be evaluated in order has to be sequenced with nested `if` statements — and past three conditions the nesting itself becomes the problem: the body drifts right, the order of evaluation is carried by indentation alone, and any shared failure path is repeated at every level. AL's `case` statement is the flat alternative. Its value sets "must be an expression or a range", so `case true of` and `case false of` accept arbitrary boolean expressions, and the statement "is evaluated, and the first matching value set executes the associated statement" — evaluation stops at the first match, which is exactly the laziness the boolean operators do not provide.
Because AL gives no short-circuit guarantee for `and` and `or`, a chain of conditions that must be evaluated in order has to be sequenced with nested `if` statements — and past three conditions the nesting itself becomes the problem: the body drifts right, the order of evaluation is carried by indentation alone, and any shared failure path is repeated at every level. AL's `case` statement is the flat alternative. Its value sets "must be an expression or a range", so `case true of` and `case false of` accept arbitrary boolean expressions, and the statement "is evaluated, and the first matching value set executes the associated statement" — evaluation stops at the first matching value set, which is exactly the laziness the boolean operators do not provide. That guarantee is stated for value sets, plural: it orders evaluation *across* separate value sets, and says nothing about the order of the individual expressions listed inside one comma-separated value set.
## Best Practice
Sequence two or three dependent conditions with nested `if`. Beyond that, switch to `case`: use `case false of` for a chain of guards where every condition must hold, letting control fall past `end` when all of them pass; use `case true of` for first-match dispatch, where each later probe runs only if the earlier ones did not match. Conditions that share an action are comma-separated into a single value set, one per line, so the chain reads as one ordered list with one failure action instead of repeating that action per condition. A value set also needs no parentheses around a comparison, unlike an operand of `and` or `or`: the AL operator hierarchy places `and` and `or` above the comparison operators, so parentheses are mandatory there and the chain fills up with them. This keeps every condition at one indentation level, makes evaluation order explicit rather than implied by nesting, and preserves the stop-at-first-match behaviour. It also aligns with the AL programming convention that more than two alternatives belong in a `case` statement rather than an `if-then-else`.
Sequence two or three dependent conditions with nested `if`. Beyond that, switch to `case`: use `case false of` for a chain of guards where every condition must hold, letting control fall past `end` when all of them pass; use `case true of` for first-match dispatch, where each later probe runs only if the earlier ones did not match. Comma-separate conditions into one value set only when every one of them is a pure, order-independent test with no side effect — a field comparison, an enum check, a bound test — so it makes no difference whether AL evaluates all of them or stops early; grouping these costs nothing and removes the repeated action. A condition that guards another, or that carries a side effect or a cost of its own — a `Get`, a `Find`, a procedure call — keeps its own value set, placed immediately after the value set it depends on, so the code relies only on the ordering the documentation actually states. A value set needs no parentheses around a comparison, unlike an operand of `and` or `or`: the AL operator hierarchy places `and` and `or` above the comparison operators, so parentheses are mandatory there and the chain fills up with them. This keeps every condition at one indentation level, makes evaluation order explicit rather than implied by nesting, and preserves the stop-at-first-match behaviour it relies on. It also aligns with the AL programming convention that more than two alternatives belong in a `case` statement rather than an `if-then-else`.
See sample: `case-true-of-for-long-condition-chains.good.al`.
## Anti Pattern
An `if` ladder four or more levels deep whose only purpose is sequencing guards. Detection: a chain of nested `if` statements with no `else`, each condition guarding the one below it, terminating in a single action or `exit`; or the same `exit`/`error` duplicated at every level of such a nested chain, purely to escape it. The second, worse form is collapsing that ladder into one `and` chain to escape the nesting — that trades indentation for a real defect, because the operands are still all evaluated. Reach for `case` instead of either.
An `if` ladder four or more levels deep whose only purpose is sequencing guards. Detection: a chain of nested `if` statements with no `else`, each condition guarding the one below it, terminating in a single action or `exit`; or the same `exit`/`error` duplicated at every level of such a nested chain, purely to escape it. The second, worse form is collapsing that ladder into one `and` chain to escape the nesting — that trades indentation for a real defect, because the operands are still all evaluated. A third, subtler form is over-applying the comma-grouping itself: putting a guard and the condition it protects — for example `Item.Get(...)` and a read of a field on that same record — into one comma-separated value set. That relies on an evaluation order within a single value set that the documentation does not state; keep them in separate value sets instead. Reach for `case` over nested `if` or a collapsed `and` chain, and keep order-dependent conditions in their own value sets within it.
See sample: `case-true-of-for-long-condition-chains.bad.al`.