Thread by @dsymetweets

I'm at #hopl and here is a potted history of the addition of what I call "computational modalities" to C# and F# - these include addition of new syntactic forms like comprehensions, or "modal" reinterpretation of existing syntactic forms.

First, in 2000-02, Todd Proebsting of MSR initiated an effort to convince the C# design team to add iterator methods, based on his experience with the Icon language, of SNOBOL fame. Iterator methods serve as a form of computed collection.

In 2005-07, those of us in F# looked at iterator methods and past history of tasksm promises, monadic and comprehension syntax to form F# async, F# sequences, F# list comprehensions and the general F# computation expressions.

During this same period, Erik Meijer, Anders Hejlsberg et al. were putting the LINQ query syntax into C#, covering an alternative comprehension syntax for IEnumerable and meta-programmed IQueryable and more, influenced by COmega protoype and functional programming

Around 2011, the F# 2.0 notion of an async modality flowed into C# 5.0, integrated with state machine compilation from Axum, TPL tasks and other influences. Likewise, F# extended computation expressions to include queries and prototyped async sequences

More recently, things have not stood still: Async has flowed out into TypeScript, Python, C++, Kotlin and other languages, C# has added async iterator methods, and F# 5.0 extended computation expressions to include the ability to express applicatives, influenced by Haskell.

I may have some details wrong, and there were many other influences and people involved. It's hard to write a comprehensive and fully accurate history, this is my recollection.

The F# applicatives work is documented here: github.com/dsyme/fsharp-p… and github.com/fsharp/fslang-…

Given the sucess of this kind of work in increasing expressivity and productivity when working with compositional computations in production languages - and its partial influence on other languages - it feels worth looking at as a historical slice through recent language design

A full history would include all comprehension and monadic notations, early coroutine additions to C++ and Ada and so on, the history of Icon iterators and SNOBOL and much more. Likely future languages will modify and extend these stories, e.g. using effect handlers etc.

Some of this is covered in this paper: tomasp.net/blog/2013/comp…, also The Early History of F# fsharp.org/history/. Perspectives from other languages may differ.

An anecdote: Todd Proebsting sent me a message saying roughly "I hear you're adding generics to C#. I don't give a damn about generics but can you teach us how to hack the compiler?" (then in C++), then visited in Cambridge. We had a great summer together, I think 2000 or 2001.