Converting a DSL to Executable F# Code On-the-Fly, Part 2

In our previous post, we started attacking the following problem: we want our application to take in raw strings, representing code written in our own, custom domain-specific language, and convert them on the fly to F# functions, so that our use can change the behavior of the application at run time. In our particular example, to keep simple, we are simply trying to inject arbitrary functions of the form f(x) = (1 + 2 * x) * 3, that is, functions that take in a float as input, and return a float by combining addition and multiplication.

As a first step, we created an internal representation for our functions, using F# discriminated unions to model functions as nested expressions. This internal DSL gave us a type-safe, general representation for any function we might want to handle. However, we are still left with one problem: what we want now is to convert raw strings into that form. If we manage to do that, we are done: our user can, for instance, write functions in our own language in a text file, and have the application pick that file and convert it to F# code it can run.

Converting a DSL to Executable F# Code On-the-Fly, Part 1

I have had a fun problem to solve for work recently. Suppose you have an application, happily running in production. Imagine that application is computing some result, based on rules. Perhaps you are computing taxes for customers, or the cost of a type of product. Now, your end-user wants the ability to change the application behavior at run time, without having to stop the application. To spice things up a bit, the end-user is not a developer. He is not particularly interested in learning our favorite programming language, and wants to specify that function in a language close to what he speaks, with no tools beyond Notepad available.

To keep it simple, for illustrations purposes, let’s imagine that our application is simply taking a number (a float), and computing something, like f(x) = 2.0 * x + 1.0. What we want is to be able to change what function is used, and replace it with any arbitrary function f, like f(x) = x * x + 3.0, or f(x) = 42.0, without modifying the code of the application itself. In this post and the next, I’ll explain how I approached it.

10 Tips for Productive F# Scripting

Scott Hanselman recently had a nice post on C# and F# REPLs, which reminded me of the time I started using F# scripts. Over time, I found out a couple of small tricks, which helped make the experience productive. I found about them mainly by accident, so I figured, let’s see if I can list them in one place! Some of these are super simple, some probably a bit obscure, but hopefully, one of them at least will make your path towards scripting nirvana an easier one…

Note: these tips are not necessarily ordered by usefulness. For that matter, there might or might not be exactly 10 of them :)

Migrating my blog to Jekyll

January is the month of new year resolutions, and one things I want to do this year is go back to more regular blogging. One obvious reason my cadence fell by the wayside is that, well, writing a book took time, and sucked some of the fun out of writing. Another, less obvious reason, was that I didn’t keep up with updates to BlogEngine.NET, and as a result, the blog itself was getting more and more broken over time. This is not the type of problem I want to spend my time on, so I figured it was time to look for something else. Long story short, I decided to take the plunge and migrate to Jekyll.

hacking together @worldbankfacts, a World Bank Twitter Bot

This is my modest contribution to the F# Advent Calendar 2015. Thanks to @sergey_tihon for organizing it! Check out the epic stuff others have produced so far on his website or under the #fsAdvent hashtag on Twitter. Also, don’t miss the Japan Edition of #fsAdvent for more epicness…

Sometime last year, in a moment of beer-fueled inspiration, I ended up putting together @fsibot, the ultimate mobile F# IDE for the nomad developer with a taste for functional-first programming. This was fun, some people created awesome things with it, other people, not so much, and I learnt a ton.

People also had feature requests (of course they did), some obviously crucial (Quines! We need quines!), some less so. Among others came the suggestion to support querying the World Bank for data, and returning results as a chart.