A useful filter can reject a legitimate production case. Keep its general rule intact, make any necessary exception explicit and narrow, explain why it exists, and protect it with a test. An example from Siddharth Pandalai’s DEV Community article shows how that principle applies to GPS distance tracking.
Why a filter needs an exception sometimes
A filter encodes a rule about which inputs should count. Passing expected test data shows that the rule works for those cases; it does not prove that every rejected input is invalid. In production, an edge case may be legitimate, and a blanket rule can discard it.
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The maintenance risk comes next: if someone adds an unexplained condition to accommodate that case, a later maintainer may mistake it for an accidental loophole and remove it. Pandalai puts the distinction plainly: “A filter with no exceptions has not met production yet. It has only met your test data.”
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Pandalai describes a filter for calculating trusted distance. In the article’s example, readings with an accuracy radius worse than 50 metres are excluded. That is the baseline for this implementation, not a universal GPS recommendation.
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A stationary vehicle can occasionally produce a still-decent fix with a wider accuracy radius. Applying the blanket threshold in that situation can leave a gap in the recorded journey. Instead of silently changing the general threshold, the example permits a narrowly defined stationary case named exceptionalStationary.
- Speed is no greater than 0.1 m/s.
- Accuracy is under 20 m.
- There is recent movement history.
All three conditions are required in the author’s code example. These values are implementation-specific constants; the article does not establish them as validated thresholds for other devices or GPS systems. The general accuracy gate still applies unless the named condition is met.
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How to make an exception maintainable
- Preserve the base rule. Start from the existing filter and identify exactly which legitimate case it rejects. Do not loosen the rule for every input just because one edge case needs different treatment.
- Define the exception narrowly. Express the relevant conditions explicitly, so the exception applies only to the situation that justifies it.
- Name it and explain why it exists. A descriptive name and a nearby comment help a future maintainer understand that the condition is intentional rather than accidental. As Pandalai writes, “An unnamed condition is indistinguishable from a bug, and it gets treated like one.”
- Add a test for the boundary. Test the ordinary filtering behavior and the exceptional case. The test should make clear both when the exception applies and when it must not apply.
Pandalai’s advice is concise: “When you find it, do not weaken the rule and do not smuggle in a condition. Give it a name, a comment with your reasoning, and a test.” The article offers this as engineering guidance, not as a quantified study of defect rates or support outcomes.
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Relax only the rule the exception is meant to address
The article also considers tracking that is paused and then resumed. During a grace window, the teleport-spike gate can be relaxed because the user may have moved while tracking was off. But the accuracy gate remains enforced.
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This illustrates an important boundary: an exception to one check is not permission to bypass neighboring checks. Keep each gate independent unless there is a specific reason to change it too. In Pandalai’s words, “An exception should be as narrow as the situation that justifies it, and no narrower than the code makes explicit.”
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A practical review checklist
- What legitimate case does the general filter reject?
- Does the ordinary rule remain in force outside that case?
- Are the exception’s conditions explicit and limited to the relevant situation?
- Does the name and comment explain the reason for the exception?
- Does a test cover both the exception and the cases where it must not apply?
- When relaxing one gate, have unrelated gates remained unchanged?
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