Know Your Limits

I just finished the nastiest debugging experience of my career–nearly 3 weeks on a single bug. After days and days of staring at code, swearing at core dumps, tailing logs, connecting to gdbserver via a multi-hop ssh tunnel from inside a secure environment, and general programmer misery, I felt like doing cartwheels when I finally figured it out, tweaked a few lines of code, and observed stability again.

Hindsight teaches me this lesson: undocumented, unhandled constraints waste enormous amounts of time and energy. If you’re interested in writing good code, you must know your limits, and you must communicate them. This especially matters when the constraints are obscure or surprising.

image credit: ericdege (Flickr)

Naive optimism

My bug seemed simple enough at first blush: Continue reading

How Enums Spread Disease — And How To Cure It

Poorly handled enums can infect code with fragility and tight coupling like a digital Typhoid Mary.

Say you’re writing software that optimizes traffic flow patterns, and you need to model different vehicle types. So you code up something like this:


enum VehicleType {

Then you press your enum into service:


if (vehicle.vt == eVTSemi || vehicle.vt == eVTTruck) {
    // These vehicle types sometimes have unusual weight, so we 
    // have to test whether they can use old bridges...
    if (vehicle.getWeight() > bridge.getMaxWeight()) {

Quickly your enum becomes handy in lots of other places as well:

if (vehicle.vt == eVTMotorcycle) {
    // vehicle is particularly sensitive to slippery roads


switch (vehicle.vt) {
case eVTTruck:
case eVTSemi:
    // can't use high-occupancy/fuel-efficient lane
case eVTMotorcycle:
    // can always use high-occupancy/fuel-efficient lane
    // can only use lane during off-peak hours


The infection from your enum is already coursing through the bloodstream at this point. Do you recognize the warning signs?

  • Knowledge about the semantics of each member of the enum are spread throughout the code.
  • The members of the enum are incomplete. How will we account for cranes and bulldozers and tractors and vans?
  • Semantics are unsatisfying. We’re saying cars are never gas guzzlers or gas savers; what about massive old steel-framed jalopies and tiny new hybrids?

A vehicle that challenges our tidy enum. Photo credit: Manila Imperial Motor Sales (Flickr)

The infection amplifies when we want to represent the enum in a config file or a UI. Now we need to Continue reading