Meetup: 100 F# Mistakes and How to avoid them

[ericharding]

Using an issue to kick off "100 F# mistakes and how to avoid them" for the July meetup.

Ignoring Immutability: F# defaults to immutable data, but developers coming from imperative languages often try to mutate variables unnecessarily. Use let bindings and embrace immutability to avoid bugs and leverage F#'s strengths.

Overusing Mutable State: While F# allows mutable state (e.g., mutable keyword or ref cells), overusing it leads to code that’s harder to reason about. Prefer pure functions and immutable data structures for safer, more predictable code.

Misunderstanding Computation Expressions: F#’s computation expressions (e.g., async, seq) are powerful but can confuse newcomers. Misusing them, like forgetting to use let! in async workflows, can lead to unexpected behavior or runtime errors.

Neglecting Pattern Matching: Pattern matching is a core F# feature, but some developers underuse it, relying instead on imperative constructs like if-else. This misses opportunities for concise, expressive code that handles complex data elegantly.

Overcomplicating Type Definitions: F#’s type system is flexible, but creating overly complex discriminated unions or record types can make code harder to maintain. Keep types simple and focused to improve readability and usability.

Ignoring the F# Core Library: Developers sometimes reinvent functions already available in modules like List, Seq, or Option. For example, using a custom loop instead of List.map or Seq.filter reduces code clarity and efficiency.

Improper Use of Option and Result Types: Newcomers may misuse Option or Result types, such as forgetting to handle None or Error cases, leading to runtime errors. Always use pattern matching or helper functions like Option.defaultValue to handle all cases.

Mixing Object-Oriented and Functional Styles Inconsistently: F# supports both paradigms, but mixing them haphazardly (e.g., using classes where functions suffice) can lead to confusing code. Stick to functional patterns unless object-oriented features are truly needed.

Not Leveraging Type Inference Properly: F#’s type inference reduces boilerplate, but developers may over-specify types or fail to provide enough type hints in complex scenarios, causing compiler errors. Balance inference with minimal annotations for clarity.

Poor Error Handling: F# encourages explicit error handling via Result or exceptions, but some developers rely on try-catch blocks excessively or ignore errors entirely. Use Result for expected errors and reserve exceptions for exceptional cases.