Functional Programming (F#) for C# Developers

The article introduces functional programming as a paradigm centered on pure functions, function composition, higher-order functions, first-class functions, immutable objects, and the absence of shared state. It contrasts declarative code with imperative code and shows how these ideas make programs easier to reason about, test, and run in parallel.

Reading notes

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• Pure functions depend only on their arguments and do not produce side effects, so a call can be replaced by its result and can support memoization.
• Function composition combines multiple functions into one, and reusable code becomes easier to build when functions can be chained.
• Higher-order functions take other functions as arguments, and first-class functions let functions be used like regular language constructs.
• Immutable objects cannot change after creation, so updates require new objects and the resulting data is thread-safe.
• With immutable data and pure functions, functional programs avoid shared state and reduce hidden interactions between functions.
• Functional code tends to be more declarative, describing what to calculate instead of how to calculate it.
• In C#, immutable types can be built with private setters and helper methods such as With to create modified copies.
• Pure functions in C# must be maintained by discipline, since the language does not enforce purity.
• Extension methods can help chain function calls in a more functional style when functions are declared separately from the data they act on.
• Immutable collections in .NET are available through System.Collections.Immutable and are created through factory methods, builders, or extension methods.
• Immutable collection operations return new instances, and structural sharing helps reduce memory allocation and garbage-collector work.
• LINQ is presented as a functional API because it is declarative, composable, and often pure when its arguments are pure.
• LINQ methods accept functions as arguments, so lambda expressions or separately defined functions can be used in Where, Select, and OrderBy.
• The article concludes that functional programming can ease parallelization, testing, and reasoning about code, and that C# can be a practical starting point before learning F#.