The benefits of using a pure functional language for creating real-world side-effecting web sites

Developing Web Applications in Haskell

There’s much more to Haskell than just buzz-words like laziness and parallelism – which are completely deserved, by the way. Having pure computations defined as side-effect-free morphisms that take and return immutable datatypes allows the compiler to do amazing optimizations. This frees you to write elegant, readable code and get near-C performance at the same time.

Runtime errors. Grepping through source to find a method you just rewrote to ensure it’s not incorrectly called somewhere. Wondering if that expression represents a String or a Boolean. Determining how a template behaves when user is nil. Unit tests. Hitting deploy and frantically browsing the site to make sure things still work. These are the hazards of a dynamic language. These are things that all go away when you use a language like Haskell.

It’s been my experience that when developing in Haskell: if it compiles, it works. I’d say, conservatively, that 93% of every bug I’ve ever written in Haskell has been caught immediately by the compiler. That’s a testament to both the compiler and the number of bugs I’m able to produce in Haskell code. It’s liberating to gain such a level of confidence in the correctness of your code simply by seeing its successful compilation.

My hope for this article is to illustrate this experience by building out a simple site in the Haskell web framework Yesod. Yesod is just one of many web frameworks in Haskell, but it’s the one I’m most comfortable with. I encourage you to check it out at as there are many features and considerations that I won’t be touching on here.


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