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With hundreds of millions of users writing formulas, Excel's side-effect-free, value-based computation model makes it a massive, unintentional functional programming environment. Recognizing this, Simon Payton Jones successfully advocated for adding Lambda functions, making Excel's formula language Turing-complete.

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Despite vastly different approaches—one based on mutating a tape, the other on pure value reduction—Alan Turing and Alonzo Church's models of computation were proven to be equally powerful. This historical context reveals that the choice between paradigms is about usability and reasoning, not computational limits.

According to Claude Code creator Boris Cherny, the single most impactful technical book for an engineer is 'Functional Programming in Scala.' While the language itself isn't widely used, the book's principles teach a new way of thinking that fundamentally improves how you approach and write code.

This approach contrasts with imperative languages where computation proceeds by mutating a state over time (e.g., a running total). Functional programming is more declarative, like a mathematical expression or a spreadsheet cell that calculates its value based on others, making it easier to reason about.

AI models are not an immediate threat to Excel because they are designed for approximation, not the precise computation required for financial and data analysis. Their 'black box' nature also contrasts with a spreadsheet's core value proposition: transparent, verifiable calculations that users can trust.

Far from its boring office reputation, Excel has a competitive scene framed as a serious eSport, complete with pro-style athlete entrances and live commentators. This cultural phenomenon highlights the software's surprising depth and passionate user base, transforming drudgery into a spectator sport.

Counterintuitively, building new AI-driven workflows can make existing software platforms more essential. For example, creating a custom financial analysis tool with AI solidifies reliance on Microsoft Excel as the core data repository, deepening its moat rather than rendering it obsolete.

Integrating AI into legacy software like Excel is a suboptimal, backward-compatible approach akin to putting a car engine in a horse carriage. The more powerful workflow is to use a native AI coding environment to generate final outputs like Matplotlib charts directly, bypassing the constraints of old UIs.

Legacy business software like Excel are "IDEs for analysts" and are doomed. The core abstraction layer is shifting from graphical interfaces with complex, hard-to-discover functions to direct, natural language interaction with agents like Claude Code, which is a fundamentally superior workflow.

Lazy evaluation allows programmers to modularly separate producer and consumer logic (e.g., an infinite data generator and a selective consumer) that would have to be merged in a strict language. For example, one can generate an infinite chess game tree and have a separate function explore only the necessary branches.

Beyond its use in formal mathematics for proof verification, Lean is a fully-featured, Turing-complete functional programming language. This dual nature allows developers to write standard code, like an autograd engine, and mathematical proofs within the same powerful system.