You work for me, Computer.

By Brandon Bloom

Control Flow Has to Flow

I’ve been experiencing MSBuild a bit at work lately. Ignoring the mortal sin of designing a programming language within XML, MSBuild fails on usability in another fundamental way: control flow has to flow.

In general, there are two key approaches to control flow. One approach is the procedural “jump” approach. This includes all manner of conditionals, loops, and more. The second approach involves functional constructs such as recursion and pattern matching. These two approaches are blended between procedural languages, functional languages, and everything in between and beyond.

The procedural and functional mechanisms can be visualized in terms of forwards and backwards. Procedural control flow moves forward in a set of steps which are executed in sequence as defined. Functional control flow begins with the results and works its way backwards pulling in bits and pieces of data and logic along the way.

MSBuild uses neither of these approaches.

That was a lie. MSBuild targets are executed in sequence procedurally. Targets can also define a “DependsOn” property which works functionally to pull in pre-requisite tasks.

The truth is that MSBuild introduces (and often requires) a very unwelcome third approach in the form of the “Condition” property. The “Condition” property is neither forwards nor backwards control flow. It is sideways. MSBuild has only one global namespace of variables (confusingly, called “properties”) which can be set or tested at any time.

With either a procedural or functional control flow, you can follow the flow simply in your text editor. In a structured procedure, you simply trace through the code pretending to be a human interpreter. In a functional method or when dealing with sub-procedures, you can trivially search for functions by name as they are called. In a sense, you are “flowing” through the code like water though pipes. You are either pumping water through or sucking water out.

With sideways control flow, you need to search for all references to every variable at every step of both the procedural and functional code flows. This is a laborious and error prone operation because after finding a relevant “Condition”, you need to analyze the forward and backwards control flow which led to that sideways control flow construct! There is no “flowing”.

Sideways control flow leads to a combinatorial explosion of forward and backwards control flows. The developer is required to keep all of these flows in their heads all at once. Meanwhile, in normal programming languages, you only ever need to deal with just two: forwards and backwards.