Pure vs. Impure Functional Languages

The concept of driving code solely through a dependency graph leads us to “pure” functional languages.

”Pure” Functional Languages (e.g., Haskell)

Languages like Haskell are often termed “pure” functional languages. In these languages, the code is, in principle, composed entirely of mathematical Expressions. Every piece of code evaluates to a value, and the program’s execution is entirely determined by the dependency graph of these expressions. The concept of “executing a statement” that doesn’t return a value or that modifies an external state doesn’t exist in the core paradigm. Everything is a calculation that produces a result, much like how 2 + 3 evaluates to 5.

This purity means:

• Code is composed, without exception, entirely of mathematical Expressions. Everything is a calculation that produces a result. The notion of a statement that is executed for its side effect, rather than its value, is absent from this paradigm.
• Driven by Dependencies: The order of evaluation is strictly dictated by what expression needs which value.

”Impure” or Hybrid Functional Languages (e.g., F#)

In contrast, languages like F# (and other ML-family languages such as OCaml) are powerful functional languages but are not “pure” in the same strict sense as Haskell. While they strongly encourage and facilitate a functional style, they also permit imperative programming constructs.

A notable area is how sequences or collections of operations might be expressed. While F# has rich functional ways to handle sequences (like Seq.map, Seq.filter), it also allows for more imperative-style iteration or sequential enumeration of steps that resemble top-to-bottom execution, rather than a strict dependency-driven evaluation for every line of code.

This means you can, for instance, have a sequence of print statements or operations with side effects in a more direct, imperative fashion, although the idiomatic F# style would often isolate these.

The Trade-offs of Purity

Working with purely functional languages can be a very enlightening experience, deeply reinforcing the concepts of dependency graphs and immutable data flow. However, this strict adherence to purity comes with its own set of challenges:

• Steeper Learning Curve: Managing everything as an expression, especially I/O and state, can initially be more complex and abstract.
• Coding Overhead: For certain tasks, particularly those that are inherently sequential or involve many side effects, the constraints of purity can sometimes lead to more verbose or less intuitive code compared to a hybrid approach. Expressing simple imperative sequences might require more sophisticated functional constructs.

In practice, many developers find that hybrid languages like F# offer a pragmatic balance, providing the benefits of functional programming (immutability, pure functions where they matter most) while still allowing for imperative escape hatches when deemed more practical or efficient for a specific problem. The choice often depends on the specific problem domain, team expertise, and performance requirements.