Or maybe, because we want to

How (and Why) Developers Use the Dynamic Features of Programming Languages: The Case of Smalltalk is an interesting analysis of the reality of dynamic programming in Smalltalk (Squeak and Pharo, really). Taking the 1,000 largest projects on SqueakSource, the authors quantitatively examine the use of dynamic features in projects and qualitatively consider why they were adopted.

The quantitative analysis is interesting: unsurprisingly a small number (under 1.8%) of methods use dynamic features, but they are spread across a large number of projects. Applications make up a large majority of the projects analysed, but only a small majority of the uses of dynamic features. The kinds of dynamic features most commonly used are those that are also supplied in “static” languages like Java (although one of the most common is live compilation).

The qualitative analysis comes from a position of extreme bias: the poor people who use dynamic features of Smalltalk are forced to do so through lack of alternatives, and pity the even poorer toolsmiths and implementors whose static analysis, optimisation and refactoring tools are broken by dynamic program behaviour! Maybe we should forgot that the HotSpot optimisation tools in Java come from the Smalltalk-ish Self environment, or that the very idea of a “refactoring browser” was first explored in Smalltalk.

This quote exemplifies the authors’ distaste for dynamic coding:

Even if Smalltalk is a language where these features are comparitively easier to access than most programming languages, developers should only use them when they have no viable alternatives, as they significantly obfuscate the control flow of the program, and add implicit dependencies between program entities that are hard to track.

One of the features of using Object-Oriented design is that you don’t have to consider the control flow of a program holistically; you have objects that do particular things, and interesting emergent behaviour coming from the network of collaboration and messages passed between the objects. Putting “comprehensible control flow” at the top of the priority list is the concern of the structured programmer, and in that situation it is indeed convenient to avoid dynamic rewriting of the program flow.

I have indeed used dynamic features in software I’ve written, and rather than bewailing the obfuscation of the control flow I’ve welcomed the simplicity of the solution. Looking at a project I currently have open, I have a table data source that uses the column identifier to find or set a property on the model object at a particular row. I have a menu validation method that builds a validation selector from the menu item’s action selector. No, a static analysis tool can’t work out easily where the program counter is going, but I can, and I’m more likely to need to know.

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