I had a really hilarious bug today where under some circumstances all the text in the display would be replaced by numbers. Small integers, each placed where the word should be.
What had happened is that I'd added a layer of indirection; where previously, after line wrapping, the data structure for a rendered paragraph was an array of pointers to the word objects, now it was an array of indices into the paragraph's word array. And I'd forgotten about one particular exotic code path. Lua was seeing the array of indices, automatically casting the ints to strings, and then using those strings instead of the word data itself...
Static types would have made this bug impossible.
...way back when, there was a really nice and thoroughly obscure language called Strongtalk; it was a Smalltalk 80 clone with optional strict types. You could annotate your classes and methods with type information. If it was there, it would be checked; if it wasn't, you got the traditional behaviour. The JIT knew about the type information and could use it to produce really fast code. It combined the ultimate dynamic language with an expressive static type system (complete with parametric polymorphism).
It was open sourced in 2006 and sank without trace. Sigh.
int[] bodies = {1, 2, 3};
for (int body : bodies) {
String formatted_body = "<p>" + body + "</p>";
callMethodWithStrArg(formatted_body);
}
bodies = [1, 2, 3]
for body in bodies:
formatted_body = '<p>' + body + '</p>'
Dynamically typed languages still have types.
I had totally forgotten that Java does it too, despite having done `""+i` lots of times as a cheap and easy and evil way to convert numbers to strings.
...I am currently rewriting a big chunk of the primary data storage to use immutable data structures, because it makes implementing Undo easier. I am having to fight the urge to redo it all in Haskell.
Ignoring trivial cases and dependent types, types tell you one of two things: "this might be correct" or "this is incorrect". Again ignoring trivial cases where exhaustive checking is possible, that's the same thing that tests tell you - and it's a hugely useful thing to be told!
Types are one mechanism for static analysis. Better contracts (nullability, valid ranges, etc) goes much further.
Compiles, runs, fails if i is out of bounds. Which means that you need to test the code that you write, and that even if you have 100% line coverage (or branch coverage or MC/DC coverage or...), it doesn't mean i won't get the wrong value.
People who claim that "once my C++/Haskell/Agda/whatever program compiles, I know it probably has no bugs" thus tempt others to mention "a false sense of security." (Agda might be going further than anyone else with proving that i cannot be out of bounds, AFAIK... though a generally undecidable problem will remain generally undecidable.)