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In my previous post on discriminated unions, I presented discriminated unions as an alternative to standard .Net classes to represent hierarchical data structures. However, in terms of data structure, discriminated unions share much more similarities with enums than they do classes – both allow you to define a set of named constants and associate some data with these constants.
The syntaxes for creating enums and discriminated unions in F# are very similar too:
Despite their apparent similarities, there are some significant differences between the two:
- Enums don’t offer a safety guarantee
- Enums only hold one piece of data
- Discriminated unions are reference types
- Enums can be used as bit flags
Now let’s take a closer look at these differences.
Enums don’t offer a safety guarantee
As enums are little more than syntactic sugar over a primitive integral type such as int, there is no guarantee that the value of an enum is valid. For instance, it’s possible to create an instance of an enum type with an integral value that is not associated with one of the named constants:
it’s easy to see how bugs can creep in when you mistakenly create enum values that don’t make any sense, especially when you’re working with enum values from external sources. Which is why it’s a good practice to check the enum values with the static Enum.IsDefined method.
Discriminated unions, on the other hand, can only be one of the defined values, any attempts to do otherwise will be met with a swift compiler error!
Enums only hold one piece of data
This one is self evident from the earlier snippet, enums only hold one piece of data but discriminated unions hold a tuple of data.
Discriminated unions are reference types
Enums are value types and instances of an enum type therefore reside on the stack as a few bytes. Discriminated unions, as do all other reference types, reside in the heap (plus a pointer on the stack whilst it’s still referenced) and need to be garbage collected when they are no longer referenced.
The implication of this is such that enums offer significant performance benefits over discriminated unions. Take the following snippet for instance, where I populate two arrays with 10 million items, one with enums and the other discriminated unions.
Averaged over three runs, the enum array took 0.048 seconds to finish whilst the discriminated union array took 1.919 seconds!
Enums can be used as bit flags
You can use an enumeration type to define bit flags, which enables an instance of the enumeration type to store any combination of the values that are defined in the enumerator list.
Hi, I’m Yan. I’m an AWS Serverless Hero and I help companies go faster for less by adopting serverless technologies successfully.
Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.
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We will start at the basics and give you a firm introduction to Lambda and all the relevant concepts and service features (including the latest announcements in 2020). And then gradually ramping up and cover a wide array of topics such as API security, testing strategies, CI/CD, secret management, and operational best practices for monitoring and troubleshooting.
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Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.
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Here is a complete list of all my posts on serverless and AWS Lambda. In the meantime, here are a few of my most popular blog posts.
- All you need to know about caching for serverless applications
- Lambda optimization tip – enable HTTP keep-alive
- You are wrong about serverless and vendor lock-in
- You are thinking about serverless costs all wrong
- Just how expensive is the full AWS SDK?
- Check-list for going live with API Gateway and Lambda
- How to choose the right API Gateway auth method
- CloudFormation protip: use !Sub instead of !Join
- AWS Lambda – should you have few monolithic functions or many single-purposed functions?
- Guys, we’re doing pagination wrong
- Top 10 Serverless framework best practices
- How to break the “senior engineer” career ceiling
- My advice to junior developers