F# – Exception handling the pattern matching way!

Pattern matching is everywhere in F#, you can use it in let bindings, in function parameters, in for loops, everywhere, and it is totally amazing. It is in my opinion one of the best features of F#, and in functional languages in general, heck, Erlang goes as far as not having an assignment operator and everything is pattern matched instead (yup, that’s right, ‘=’ is the pattern match operator in Erlang, not assignment!). Once you’ve had a chance to work with it you’ll understand why those functional folks love it so much!

In terms of exception handling, consider this piece of code in C#:

Notice that the switch statement allows you to apply the same handler to multiple cases but not the catch statements, which leads to duplicated exception handling code. Wouldn’t it be nice if you don’t have to keep repeating yourself?

Now, let’s see how pattern matching in F# lets us do just that:

Pretty neat, right? Not only are you able to group handling code for different exceptions, you can also use wild card ( _ ) and when guards (e.g. | _ as ex when ex.Message.Contains(“Oops”) to match any exception whose message contains the term ‘Oops’) too, which give you a very fine control over which handler is used for what exceptions.

As the patterns are matched from top to bottom, if you move the wildcard pattern to the top you’ll receive a set of gentle warnings from the compiler letting you know that doing so will mean that each of the subsequent patterns will never be matched.

It’s also worth noting that whilst F# has try-catch and try-finally expressions, there is no try-catch-finally expression.

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Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

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