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Style 24 – Quarantine
- Core program functions have no side effects of any kind, including IO.
- All IO actions must be contained in computation sequences that are clearly separated from the pure functions.
- All sequences that have IO must be called from the main program.
This style is similar to The One (aka Monads) style we saw earlier on. The notable difference here is that only the side-effecting IO code need to be contained, so we can interpret this as the IO Monad style.
Taking inspiration from the computation expression we built for The One style, we can make some modifications here.
We’ll start by declaring a generic IO<‘a> type that will encapsulate an IO action. Then we can build a computation expression around this wrapper type.
Unfortunately do is already a reserved keyword in F#, so we’ll have to make do with do’ instead (see what I did there? )
Now we can add a couple of helper types and methods to wrap around IO operations – such as reading from a file or printing to console – in our IO<‘a> type.
We need to read from files in order to extract words from Pride and Prejudice and to remove stop words, so both functions use our do’ computation expression. Notice that we’re using the aforementioned File.ReadAllText helper method, which returns an IO<string> so our do’ computation expression can bind to.
Other pure functions can stay as they are.
But, our main program will need to be contained since it needs to perform IO operations via the extractWords and removeStopWords functions.
Note that no work has actually been done yet.
So far, we have merely composed together what actions (IO and otherwise) we will perform when this program is run. To complete the puzzle we need one more thing – a mechanism to execute these side-effecting programs.
Something like this IO module perhaps?
which we can use to execute our mainProgram.
Now, the IO operations (including printing to the console) will be performed along with other pure functions.
You can find the source code for this exercise here
Hi, I’m Yan. I’m an AWS Serverless Hero and I help companies go faster for less by adopting serverless technologies successfully.
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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.
Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!
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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