Tuesday, November 17, 2009

Where do you benefit from dynamic typing?

by skeet via Jon Skeet: Coding Blog on 11/17/2009 7:31:48 AM

Disclaimer: I don't want this to become a flame war in the comments. I'm coming from a position of ignorance, and well aware of it. While I'd like this post to provoke thought, it's not meant to be provocative in the common use of the term.

Chapter 14 of C# in Depth is about dynamic typing in C#. A couple of reviewers have justifiably said that I'm fairly keen on the mantra of "don't use dynamic typing unless you need it" – and that possibly I'm doing dynamic typing a disservice by not pointing out more of its positive aspects. I completely agree, and I'd love to be more positive – but the problem is that I'm not (yet) convinced about why dynamic typing is something I would want to embrace.

Now I want to start off by making something clear: this is meant to be about dynamic typing. Often advocates for dynamically typed languages will mention:

• REPL (read-eval-print-loop) abilities which allow for a very fast feedback loop while experimenting
• Terseness – the lack of type names everywhere makes code shorter
• Code evaluated at execution time (so config files can be scripts etc)

I don't count any of these as benefits of dynamic typing per se. They're benefits which often come alongside dynamic typing, but they're not dependent on dynamic typing. The terseness argument is the one most closely tied to their dynamic nature, but various languages with powerful type inference show that being statically typed doesn't mean having to specify type names everywhere. (C#'s var keyword is a very restricted form of type inference, compared with – say – that of F#.)

What I'm talking about is binding being performed at execution time and only at execution time. That allows for:

• Duck typing
• Dynamic reaction to previously undeclared messages
• Other parts of dynamic typing I'm unaware of (how could there not be any?)

What I'm interested in is how often these are used within real world (rather than demonstration) code. It may well be that I'm suffering from Blub's paradox – that I can't see the valid uses of these features simply because I haven't used them enough. Just to be clear, I'm not saying that I never encounter problems where I would welcome dynamic typing – but I don't run into them every day, whereas I get help from the compiler every day.

Just as an indicator of how set in my statically typed ways I am, at the recent Stack Overflow DevDays event in London, Michael Sparks went through Peter Norvig's spelling corrector. It's a neat piece of code (and yes, I'll finish that port some time) but I kept finding it hard to understand simply because the types weren't spelled out. Terseness can certainly be beneficial, but in this case I would personally have found it simpler if the variable and method types had been explicitly declared.

So, for the dynamic typing fans (and I'm sure several readers will come into that category):

• How often do you take advantage of dynamic typing in a way that really wouldn't be feasible (or would be very clunky) in a statically typed language?
• Is it usually the same single problem which crops up regularly, or do you find a wide variety of problems benefit from dynamic typing?
• When you declare a variable (or first assign a value to a variable, if your language doesn't use explicit declarations) how often do you really either not know its type or want to use some aspect of it which wouldn't typically have been available in a statically typed environment?
• What balance do you find in your use of duck typing (the same method/member/message has already been declared on multiple types, but there's no common type or interface) vs truly dynamic reaction based on introspection of the message within code (e.g. building a query based on the name of the method, such as FindBooksByAuthor("Josh Bloch"))?
• What aspects of dynamic typing do I appear to be completely unaware of?

Hopefully someone will be able to turn the light bulb on for me, so I can be more genuinely enthusiastic about dynamic typing, and perhaps even diversify from my comfort zone of C#…