Programming @programming.dev 𝕊𝕚𝕤𝕪𝕡𝕙𝕖𝕒𝕟 @programming.dev 2y ago

A quick tip for better variable names

Whenever you store a value that has a unit in a variable, config option or CLI switch, include the unit in the name. So:

maxRequestSize => maxRequestSizeBytes
elapsedTime => elapsedSeconds
cacheSize => cacheSizeMB
chargingTime => chargingTimeHours
fileSizeLimit => fileSizeLimitGB
temperatureThreshold => temperatureThresholdCelsius
diskSpace => diskSpaceTerabytes
flightAltitude => flightAltitudeFeet
monitorRefreshRate => monitorRefreshRateHz
serverResponseTimeout => serverResponseTimeoutMs
connectionSpeed => connectionSpeedMbps

EDIT: I know it’s better to use types to represent units. Please don’t write yet another comment about it. You can find my response to that point here: https://programming.dev/comment/219329

8 comments

That seems akin to commenting. The problem with this approach is that text is not code. It's very easy to forget to change text. In that case it becomes the worst of both worlds, you have a variable name that actually misleads you.

Much safer than this is to encode this kind of information into the code itself in such a way the program won't compile of the types are incorrect.
- I understand what you mean, and I even agree with it, but just to be a little pedantic, variable names are code, or at least they are more code than comments or docs.
  
  But yes, encoding units into the type system is a much better solution. It doesn't work however for config options, environment variables or CLI switches.
I think some of the modern languages handle this pretty well. Rust has algebraic data types thanks to its brilliant use of enums. Go has a similar type system. Taking the elapsedTime example from the post, for solving this duration related problem, a Rust programmer would use Duration::from_millis(millis) or Duration::from_secs(secs) and forget about the unit. It's a duration, that's what you wanna care about.
Those are just types. You shouldn't write types in the names. It's called Hungarian Notation, but it's just redundant. If you need to check the type of a variable, hover over it and your IDE should tell you that temperatureThreshold is type DegreesCelsius. No need to add extra cruft. There's also a question of how specific everything needs to be.

It's also especially problematic if you later refactor things. If you change units, then you have to rename every variable.

Plus, variables shouldn't really be tied to a specific unit. If you need to display in Fahrenheit, you ideally just pass temperatureThreshold and it converts types as needed. A Temperature type that that has degreesF() and degreesC() functions is even cleaner. Units should just be private to the type's struct.
- I absolutely agree. But:
  
  sometimes you need to modify existing code and you can't add the types necessary without a giant refactoring
  
  you can't express units with types in:
  
  JSON/YAML object keys
  
  XML tag or attribute names
  
  environment variable names
  
  CLI switch names
  
  database column names
  
  HTTP query parameters
  
  programming languages without a strong type system
  
  Obviously as a Hungarian I have a soft spot for Hungarian notation :) But in these cases I think it's warranted.
- There are plenty of times where the type is just something generic like an integer and making a wrapper type is not worth the effort and this is a useful approach.
In languages with static and convenient type systems, I try to instead encode units as types. With clever C++ templating, you can even get implicit conversions (e.g. second -> hour) and compound types (e.g. meter and second types also generate m/s, m/s^2 and so on).
- IIRC F# even has built-in support for units.

8 comments