There is a cost associated with eliminating this class of errors using static typing. The cost is that you're restricted to a set of statements that the type checker can verify to be correct. Writing code for the benefit of the type checker is often at odds with writing it in a way that conveys the meaning best to the human reader. This is necessarily less expressive than the dynamic approach. Code written in dynamic languages tends to do a better job of expressing its intent because it can be written in a more direct fashion.
Here's a concrete real world example of what I'm talking about:
>When I first wrote the core.async go macro I based it on the state monad. It seemed like a good idea; keep everything purely functional. However, over time I've realized that this actually introduces a lot of incidental complexity. And let me explain that thought.
>What are we concerned about when we use the state monad, we are shunning mutability. Where do the problems surface with mutability? Mostly around backtracking (getting old data or getting back to an old state), and concurrency.
>In the go macro transformation, I never need old state, and the transformer isn't concurrent. So what's the point? Recently I did an experiment that ripped out the state monad and replaced it with mutable lists and lots of atoms. The end result was code that was about 1/3rd the size of the original code, and much more readable.
>So more and more, I'm trying to see mutability through those eyes: I should reach for immutable data first, but if that makes the code less readable and harder to reason about, why am I using it?
So really what you're doing with static typing is trading one set of problems for another. This is perfectly fine if those are the kinds of problems you prefer to deal with, but it's important to recognize that you are making a trade off as opposed to getting something for free here.
In my opinion you get more than just "eliminating a class of bugs", in my (arguably limited) forays into functional programming languages I really liked the type-guided programming.
One aspect is "I refactored the code, fixed all the type-errors and everything works", another is "I don't know, what should I write here, compiler, tell me!" with typed-holes, along-side some nice search, such as hoogle (or elm's fancy search [1]) In simmilar fashion, I remember Elm was enforcing a version bump, if you break package public api.
On the other hand, you definitely are replacing one set of problems for a different set, and it is up to you to decide what kind of problems you like solving better.
For me, access to fast immutable data-structures seem to have the best return-on-investment, and easiest to introduce (i.e. even Javascript or Python have somewhat decent libraries for these).
With Clojure the approach is to use REPL driven development. It's tightly integrated with the editor, and any time you write a function you run it to see that it's doing what you intended. Because you're evaluating code as you're writing it, there's generally no confusion regarding what the code is doing. [1]
Meanwhile, immutability as the default makes it natural to structure applications using independent components. This helps with the problem of tracking types in large applications as well. You don't need to track types across your entire application, and you're able to do local reasoning within the scope of each component. You make bigger components by composing smaller ones together, and you only need to know the types at the level of composition which is the public API for the components.
Spec [2] is a contract system that's often used to define API boundaries in Clojure. I find it provides an advantage over static typing because it directly focuses on semantic correctness. For example, consider a sort function. The types can tell me that I passed in a collection of a particular type and I got a collection of the same type back. However, what I really want to know is that the collection contains the same elements, and that they're in order. This is difficult to express using most type systems out there, while trivial to do using Spec.
Nice, if I ever will have the pleasure of working in clojure, I will remember the re-find thing you mentioned here :-)
But to show more of I really liked when developing with types, take a look at this slightly contrived example [1]. I once tried to refactor a medium size haskell code-base, and ability to just ask the compiler "Hey what should I put in here?" really made my life easier :)
Yeah, there are nice aspects of having static typing as well. Haskell was the first FP language I've used actually, and it was a lot of fun. Eventually I ended up working with Clojure professionally, and I don't find that one approach or the other is strictly better. There are pros and cons to each, and they're both productive.
Here's a concrete real world example of what I'm talking about:
>When I first wrote the core.async go macro I based it on the state monad. It seemed like a good idea; keep everything purely functional. However, over time I've realized that this actually introduces a lot of incidental complexity. And let me explain that thought.
>What are we concerned about when we use the state monad, we are shunning mutability. Where do the problems surface with mutability? Mostly around backtracking (getting old data or getting back to an old state), and concurrency.
>In the go macro transformation, I never need old state, and the transformer isn't concurrent. So what's the point? Recently I did an experiment that ripped out the state monad and replaced it with mutable lists and lots of atoms. The end result was code that was about 1/3rd the size of the original code, and much more readable.
>So more and more, I'm trying to see mutability through those eyes: I should reach for immutable data first, but if that makes the code less readable and harder to reason about, why am I using it?
https://groups.google.com/forum/#!topic/clojure/wccacRJIXvg
Another example of something that's trivial in a dynamic language, but difficult to do in a static one would be Ring middleware: https://github.com/ring-clojure/ring/wiki/Middleware-Pattern...
So really what you're doing with static typing is trading one set of problems for another. This is perfectly fine if those are the kinds of problems you prefer to deal with, but it's important to recognize that you are making a trade off as opposed to getting something for free here.