The Early History of F# DON SYME, Microsoft, United Kingdom Shepherd: Philip Wadler, University of Edinburgh, UK This paper describes the genesis and early history of the F# programming language. I start with the origins of strongly-typed functional programming (FP) in the 1970s, 80s and 90s. During the same period, Microsoft was founded and grew to dominate the software industry. In 1997, as a response to Java, Microsoft initiated internal projects which eventually became the .NET programming framework and the C# language. From 1997 the worlds of academic functional programming and industry combined at Microsoft Research, Cambridge. The researchers engaged with the company through Project 7, the initial effort to bring multiple languages to .NET, leading to the initiation of .NET Generics in 1998 and F# in 2002. F# was one of several responses by advocates of strongly-typed functional programming to the “object-oriented tidal wave” of the mid-1990s. The 75 development of the core features of F# 1.0 happened from 2004-2007, and I describe the decision-making process that led to the “productization” of F# by Microsoft in 2007-10 and the release of F# 2.0. The origins of F#’s characteristic features are covered: object programming, quotations, statically resolved type parameters, active patterns, computation expressions, async, units-of-measure and type providers. I describe key developments in F# since 2010, including F# 3.0-4.5, and its evolution as an open source, cross-platform language with multiple delivery channels. I conclude by examining some uses of F# and the influence F# has had on other languages so far. CCS Concepts: • Software and its engineering ! General programming languages; • Social and pro- fessional topics ! History of programming languages. Additional Key Words and Phrases: Programming Languages, Functional Programming, Object-oriented Programming, Pattern Matching, Asynchronous Programming, Type Providers, Units of Measure, Dimensions, F# ACM Reference Format: Don Syme. 2020. The Early History of F#. Proc. ACM Program. Lang. 4, HOPL, Article 75 (June 2020), 58 pages. https://doi.org/10.1145/3386325 Author’s address: Don Syme, Microsoft, United Kingdom, [email protected]. Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for third-party components of this work must be honored. For all otheruses, contact the owner/author(s). © 2020 Copyright held by the owner/author(s). 2475-1421/2020/6-ART75 https://doi.org/10.1145/3386325 Proc. ACM Program. Lang., Vol. 4, No. HOPL, Article 75. Publication date: June 2020. 75:2 Don Syme Contents Abstract 1 Contents 2 1 Introduction3 2 What is F# in 2020?4 3 Background: Languages, Programmability at Microsoft and the Creation of .NET4 4 Background: Strongly Typed Functional Programming through the 1990s—Calculi, Miranda, OCaml, Haskell and Pizza7 5 Project 7 and .NET Generics 10 6 The Decision to Create F# 13 7 Early F#—2002–2003 18 8 Early F#—Release 20 9 F# 1.0, 2004–2006—Overview 22 9.1 F# 1.0—Pipelines 23 9.2 F# 1.0—Tackling Object Programming 24 9.3 F# 1.0—Improving the Functional Core: Initialization Graphs 26 9.4 F# 1.0—Improving the Functional Core: Overloaded Arithmetic 27 9.5 F# 1.0—Improving the Functional Core: Active Patterns 28 9.6 F# 1.0—Improving the Functional Core: First-Class Events 30 9.7 F# 1.0—Improving the Functional Core: async/await 30 9.8 F# 1.0—Improving the Functional Core: Computation Expressions 32 9.9 F# 1.0—Meta-programming 35 9.10 F# 1.0—Improving the Functional Core: Indentation-Aware Syntax 36 9.11 F# 1.0—IDE Tooling 36 10 Finance and Functional: Microsoft Commits to F#, 2007 37 11 F# 2.0—2007 to 2010 38 12 F# 2.0—Units of Measure 40 13 Type Providers and F# 3.0 41 14 .NET, F# and the Shift to Cloud and Mobile Computing 44 15 A New Dawn for F#, C# and .NET: Open and Cross-Platform, At Last! 45 16 The F# Compiler as a Component 46 17 The F# Community and the F# Software Foundation 46 18 .NET Core: Microsoft take C#, F# and .NET Cross-Platform 47 19 F# for Mobile 48 20 F#, JavaScript and Full Stack Programming 48 21 Retrospective 48 21.1 F#’s Influence 49 21.2 Mistakes and Questions 51 22 Conclusion 52 Acknowledgments 52 References 53 Proc. ACM Program. Lang., Vol. 4, No. HOPL, Article 75. Publication date: June 2020. The Early History of F# 75:3 1 INTRODUCTION The history of the F# programming language is an arc drawn from the 1970s to the present day. Somewhere, back in the early 1970s, an idea was born in the mind of Robin Milner and his colleagues Lockwood Morris and Malcolm Newey of a succinct, fully type-inferred functional programming language suitable for manipulating structured information [Gordon 2000]. Building on the tradition of LISP (and indeed using LISP as their implementation vehicle), this language became ML—Meta Language—and is the root of a tradition of “strongly typed functional programming languages” that includes Edinburgh ML, Miranda, Haskell, Standard ML, OCaml, Elm, ReasonML and PureScript. F# is part of this family. The history of Standard ML has been told elsewhere [MacQueen 2015]. ML-family languages are often associated with formalism, a theme I explore later in this article. However, a primary concern of Milner and co. from the outset was pragmatic usability. This group needed their language for a specific purpose: to succinctly and accurately program the proof rules and transformations (“tactics”) of a theorem proving system called LCF, at that time on PDP-10 machines. Pragmatic choices included the use of mutable state (to allow proof state to be stored in an interactive system) and a type inference system (later called Hindley-Milner or Damas-Milner type inference), allowing the code for derived tactics to be both succinct and automatically generalized. A similar theme of pragmatism ran through later ML dialects as well, including OCaml [Leroy 2014], witnessed by both the language design and tooling such as the OCaml C Foreign Function Interface (FFI). Rolling forward, to the present day, key ideas stemming from the 1970s are at the core of the F# language design and central to the day-to-day experience of using the language. Like all ML-family languages: • The core paradigm supported by F# is still strongly typed functional programming; • The core activity of F# is still defining types (type X) and functions (let f x = ...) and these declarations are type-inferred and generalized automatically; • F# still aims to support a mode of programming where the focus is on the domain being manipulated rather than on the details of programming itself. Today, books are published which extoll the virtues of F# for “Domain Driven Design” [Wlaschin 2018]. This is not so far removed from the early role of ML where the “domain” was the symbolic representation of terms and theorems of the LCF logic. The “spirit” of ML is very much alive in F#, as it was always intended to be. The leap from 1970s to the present day spans a period of massive change in the computing industry: we have shifted from PDP-10s to cloud systems, from punch cards to mobile phones, from edit-line to tooling-rich IDEs, from small to massive storage, from no-network to ubiquitous network. This article tells the story of how F# developed, the industry and academic contexts in which this occurred, the immediate influences on the language and its distinctive contributions. The story intersects with many other histories in programming language design, including the complex histories of functional programming, object-oriented programming, type systems, runtime design, operating systems and open source software, and emphasis is placed on the genesis of F# as one of several reactions to the “object-oriented tidal wave” of the early 1990s. The story is necessarily incomplete and told largely from the personal perspective of the author, the designer of the language, and I apologize for that. Where references are not provided the text is offered as source material based on the recollection of the author.1 I have started with the core idea of the ML-family of programming languages: type-safe, succinct, accurate, domain-oriented functional programming. From my perspective, this idea has “held 1Additional historical material on F# and the ML family of languages has been collected and presented by Rachel Blasucci, see [Blasucci 2016]. Proc. ACM Program. Lang., Vol. 4, No. HOPL, Article 75. Publication date: June 2020. 75:4 Don Syme strong, held true” throughout this era of change. Whether that is through obstinacy, coherence or coincidence is something I leave the reader to assess. 2 WHAT IS F# IN 2020? In 2020, F# is described on its documentation pages as “a functional programming language that runs on .NET.” The F# language guide [Carter 2020] calls out the following major features of the language, providing useful clarity about what the language is today: • functions and modules • pipelines and composition • lists, arrays, sequences • pattern matching • active patterns • type inference • recursive functions • quotations • record types, discriminated union types • option types • units of measure • object programming • asynchronous programming • computation expressions • type providers The documentation continues with an explanation of the main tooling and libraries available for F# programming, including • cross-platform compilation and execution; • the primary F# and .NET libraries; • web, mobile and data programming toolkits; • editing tools from Emacs to Visual Studio, VSCode and JetBrains Rider; • how to use F# with the cloud platform of the company providing the documentation.