5 Common Mistakes to Avoid in PureScript

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Common Pitfalls in PureScript
Common Pitfalls in PureScript

PureScript, a strongly-typed, purely-functional programming language that compiles to JavaScript, is gaining traction in the functional programming community. It offers robust type-checking and elegant functional constructs, making it a favorite for those who love Haskell but need to work in a JavaScript environment. However, with its unique features come common pitfalls that newcomers and even experienced developers might encounter. In this blog post, we'll dive into the five most common mistakes made when working with PureScript and how to avoid them.

1. Misunderstanding Type Classes

Type Classes Misconception:

In PureScript, type classes are a powerful feature used to define functions that can operate on a variety of types. A common mistake is to confuse them with classes in object-oriented programming.

Avoidance Strategy:

Understand that type classes in PureScript are more about specifying a set of functions that a type must support, rather than encapsulating data and behavior like classes in OOP. To avoid this mistake, spend time learning the fundamentals of type classes, including their declaration and implementation. Also, get familiar with common type classes in PureScript, such as Eq, Ord, and Functor.

2. Overusing or Misusing Monads

Monads Mismanagement:

Monads are a central concept in PureScript, often used for managing side effects and building complex functional pipelines. A typical mistake is either overusing them where simpler constructs would suffice, or misunderstanding their purpose and usage.

Avoidance Strategy:

Before reaching for a monad, evaluate if a simpler abstraction like Functor or Applicative would suffice. When using monads, ensure you understand the monad's purpose. For instance, Maybe is for optional values, Either for error handling, and Aff for asynchronous effects. Practice writing monadic code in small, isolated examples to build intuition.

3. Ignoring or Misapplying Purity

Purity Pitfalls:

PureScript enforces pure functional programming, meaning functions should have no side effects. A common mistake is inadvertently introducing impurity, for instance, by interacting with external state.

Avoidance Strategy:

Embrace PureScript's pure functional nature. When side effects are necessary, use appropriate constructs like Effect or Aff. Always keep the core logic of your application pure, and isolate side effects to the boundaries. This approach not only respects the language's paradigms but also leads to more predictable and testable code.

4. Inefficient Data Structures

Data Structures Dilemma:

Using inappropriate data structures can lead to performance bottlenecks. In PureScript, it's easy to default to familiar structures like lists, even when they're not the most efficient choice.

Avoidance Strategy:

Understand the strengths and weaknesses of different data structures. For example, use arrays when you need random access or efficient updates, and use lists for scenarios where you're primarily doing sequential processing or transformations. Explore PureScript's libraries for data structures and choose the one that best suits your use case.

5. Not Leveraging the Ecosystem

Ecosystem Exploration Error:

PureScript has a rich ecosystem of libraries and tools, but developers sometimes reinvent the wheel instead of leveraging existing solutions.

Avoidance Strategy:

Before implementing a solution, explore what's available in the PureScript ecosystem. Use libraries like purescript-prelude for basic tools, purescript-aff for asynchronous programming, and purescript-halogen for UI development. Also, participate in the community through forums and chats to stay informed about best practices and new tools.


PureScript offers a robust environment for functional programming, but it comes with its nuances and complexities. By avoiding these common mistakes, you can harness the full power of PureScript, writing more efficient, readable, and maintainable code. Remember, the key to mastering PureScript lies in understanding its core principles, such as pure functions, type classes, and monads, and effectively leveraging its rich ecosystem of tools and libraries. As you grow more accustomed to PureScript's paradigms, you'll find it a rewarding language that significantly enhances your functional programming capabilities.

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