Things I Was Wrong About: Types

Same here! I've started with C++ and Java, learned to hate excessive typing, went through a long period of dynamic typing, and now I'm at the point you and the the author are.

I still code a lot of Common Lisp on the side, but my Lisp code now looks entirely different than it looked just 3 years ago. The language standard does support optional typing declarations, and there's an implementation (SBCL) that makes use of it to both optimize code and provide some static typechecking at compile time (with type inference). So my Lisp code now is exploiting this, and is littered with type declarations.

However, the CL type system is very much lacking compared to Rust or Haskell. I'm hoping one day someone will make a statically, strongly typed Lisp that still doesn't sacrifice its flexibility and expressive power. I'd jump to that in an instant.

I can second the experience. I write a lot of Common Lisp, and these days it's typed Common Lisp for me. It adds very little overhead in terms of code writing speed, but continuously stops me from making stupid mistakes (like e.g. forgetting a function I'm calling returns a sequence and treating it as a scalar value). My comfort of writing is much better, because I spend less time in interactive debugger hunting my own typos.

All lisps I used could be statically typed, I think? Does elisp used typep?

For small projects, hacks or emacs macros types are not useful but the possibility of adding types is there.

[Edit:and it's useful when your finite state machine grows to much]

The view I've heard expressed is that deep thought on a piece of code reduces bugs. Whether that takes the shape of religious TDD, rigorous proofs or detailed type design doesn't make such a lot of difference.

I used to be fully bought into types, but I've since realised that they have a number of downsides that in many cases more than offset their benefits:

1. Ergonomic typesystems require a lot of work to happen at compile time and slow down the iteration time (one of the more important things for programming in my view). In my view, saving the source and seeing the result almost immediately in a browser is one of the big advantages of web development.

2. Types are almost always written in a second, much less powerful DSL and then sprinkled distractingly through the code that actually does the work. I prefer the way Haskell does this- separate the type signature out onto at least a separate line rather than mashing the two different languages together.

3. Higher levels of abstraction tend to become very hairy in many type systems (although not all). This ends up just meaning that people who like types often restrict themselves (unconciously) to less abstract programming. They spot the time they're saving by avoiding some kinds of bugs, but they don't see the time they're wasting by being unable to talk at a higher level of abstraction. Another way this shows itself is that types are very rarely first class objects in strongly typed languages, making it very difficult to create code that operates on types, or understands types.

4. Type systems open up opportunities for type driven architecture astronauting, which is just yet another way you can go down an unproductive rabbit hole. There was an interesting study done on different teams solving problems with different languages. The differences of different teams within the same language was much bigger than between languages, but the team that made slowest progress (and without particularly having an unusually low number of bugs) was the team that leant the hardest into encoding everything in the type system.

5. Type systems encourage code generation build pipelines, which again slows iteration time and makes everyones life miserable.

6. Type systems reflect a incorrect model of the world - user input, network input, file system data is not typed. The misery that I've had with some web server frameworks that refuse to acknowlege that they don't know every possible thing that the web client might send them and are able to slot it into a predefined type. I think this is the same kind of error that we made with OO systems - thinking that we could fit the world into a predefined inheritance hierarchy.

7. Type systems encourage a static view of the world. The types of things can change under you, dynamically, (e.g. the structure of a table in a database), but in most typed languages you can't cope with that correctly without shutting down and deploying entirely new code.

8. Related to that, it's hard to imagine using a strongly typed language with the live image approach of smalltalk or sometimes used by lisp systems. This means that the popuarity of strongly typed languages is killing valuable and interesting approaches to building complex systems that emphasise observability, interaction and iteration as a way of understanding them.

There are genuine advantages to typed languages, but many of the advantages touted as being unique to typed languages can be provided by advanced linting and IDEs (intellij was surprisingly capable on plain JS + jsdoc). You can also ameliorate some of the disadvantages of untyped languages while keeping the benefits by deliberately programming in a fail-fast way.

I'm sure that type systems have their place. The research I've come across on empirical studies suggests that while there may be positive effects they are small, which does not at all mesh with the extreme partisanship I generally observe. Yes, type systems gain you something, but there seems much less awareness of what you lose.

The correct thing is imho to pass an immutable SomeType or an interface that only exposes the parts of SomeType necessary for the calculation and doesn’t allow mutation of the object.

Of course you don’t send around references to mutable objects and of course you only send to a function just what it needs - but that’s regardless of type system.