> Types guarantee that your code executes as you expect, and prevent you from writing incorrect code in the first place.

If you write a Rational class and implement it's + method as

    def +(other: Rational):Rational = new Rational(0)

then no amount of static typing will save you. At least without dependent types.

Static typing gives you a knowledge about what kind of object you're dealing with without the need to run the code. It's almost completely useless if the "kinds" supported in type system are as broad as 'int', but it does help quite a lot if the type system let's you differentiate between cm and inches.

It still won't help you at all if you have a logic error - and this is where unit tests are valuable.

Personally I find a mixture of Typed Racket and Racket with contracts and unit tests to be the right thing when it comes to producing as error-free programs as possible. I was also pleasantly surprised with Smalltalk, which has no static typing, but because it runs all the time and every piece of code becomes "live" the moment it's written you can use introspection in place of static typing with very similar benefits.

Anyway, static typing is a valuable tool and one way of making software less wrong but it's important to know there are other tools and to use them all when it makes sense.

> then no amount of static typing will save you. At least without dependent types.

Sure. Static typing also doesn't help with misunderstanding requirements, or a faulty test in a recursive function. But strong typing, especially in languages where there is no such thing as a NullPointerException, does eliminate entire class of problems. On the other hand, with dynamic languages, you are often a typo away from a runtime error.

> usually work correctly after they compile

I still don't buy in to this trope.

Q: What is true of every single bug in production?

A: It passed both your type checker and your tests.

That's a tautology. By that reasoning, don't bother writing any tests, because all your bugs in production will get past them anyway!

My bugs got past my type-checker, but only 100 of them. You have 1000 because the 900 my type checker caught, your dynamic language didn't. Then you go and write unit tests for them, and spend a day writing code that my compiler generated automatically for me.

> By that reasoning ...

My reasoning in no way implies that. I simply said that I'm sick of this argument that "if it compiles, it works!" Nobody who knows what they are talking about actually believes that, especially not people who design and build statically typed languages!

If you can provide statistics on bugs being an order of magnitude higher in dynamic languages to back up your figures here, it would be a far more convincing argument. I'm sure type safety catches a few bugs and prevent some from being made, but does it catch as many as you think?

Look at it the other way. In fact, you can do a blind test. Try writing in a strongly, (well[0]) statically-typed language for a while, and count the number of times your code fails to compile due to a type error.

Each of those would likely have been a runtime error in a language like Python.

Just earlier today, I had to write a short Python script (I normally write Go), and I was bitten by this. I'm used to the compiler telling me up-front when I try to concatenate a string and an integer, or when I misspell a variable name.

Yes, there are other ways around this (and proofreading your code before you write it is a good practice, in addition to tests). But static typing can make development much faster[1].

Even if you claim that you would have caught all of those bugs in unit/integration/system tests (which I simply don't believe - no real-world project has test coverage that's that good), it's much better to catch bugs at compile-time, because (unless you're writing C++) your tests take much longer to run than your code does to compile.

[0] I qualify this to rule out C, which has weak typing as well as memory management issues to complicate things, and Java, which just has a terrible type system period.

[1] In some sense this is all a moot point, because any difference in development speed and/or code correctness between statically-typed and dynamically-typed languages is dwarfed by one's familiarity with the languages in question. The only real way to test this directly would be to use a statically-typed language that allows disabling of all compile-time type checks as a compiler flag or pragma, but few languages do this in a way that'd be straightforward to do a meaningful blind test.

Look at it the other way. In fact, you can do a blind test. Try writing in a strongly, (well[0]) dynamically-typed language for a while, and count the number of times your code fails at the REPL due to a type error.

Each of those would likely have been a compile time error in a language like Haskell.

Just earlier today, I was writing some C++ (I normally write Clojure) and I was bitten by this. I'm used to evaluating code in the REPL as I work, telling me up front when I try to concatenate a string and an integer, or when I misspell a variable name.

Yes, there are other ways around this (and proofreading your code before you write it is a good practice, in addition to tests). But dynamic typing can make development much faster[1].

Even if you claim that you would have caught all of those bugs in a type system (which I simply don't believe - no real-world project has types that are that precise), it's much better to catch bugs at run-time, because (unless you're writing Go) your compiler takes much longer to run than your REPL does to eval.

[0]: I qualify this to rule out Python, which has weak typing as well as mutability issues to complicate things, and JavaScript, which just has a terrible REPL period.

[1]: In some sense this is all a moot point, because any difference in development speed between statically-typed and dynamically-typed languages is dwarfed by one's familiarity with the languages in question. The only real way to test this would be to use a dynamically-typed language that has an optional type system as an external tool, luckily quite a few languages do this in a way that'd be straightforward to do a blind test (Typed Racket, Typed Clojure, Erlang w/ Dialyzer).

> Each of those would likely have been a compile time error in a language like Haskell.

Why do you consider this a hindrance? Haskell has a lot of issues, but that's really not one of them. You can write a Haskell program and be relatively confident that it's going to work right in a way you simply won't be with, eg, Java. And you don't have to wait until compiling. With ghc-mod and vim/emacs configured, you'll be notified upon saving if you blew up something in the current file. You don't need to run your code in a REPL to find out about it, although GHCi is pretty good. You spend a lot less time figuring out what goes wrong at runtime, although I'll grand you that the tooling is poor enough that you have a bad time doing when it happens.

> I qualify this to rule out Python, which has weak typing

Excuse me? You rule out Python due to weak typing and mutability but disqualify Javascript due to its REPL? This must be sarcasm. I have no comment about Javascript's REPL (which one? node's?), but Python is most certainly not weakly typed, not in the way "weak typing" is commonly understood. As for mutability, they are comparable (except that Python has at least the good grace to throw an exception at runtime if you attempt to access an attribute which does not exist).

You can have a REPL in a statically typed language too---Haskell has one.

GHC's REPL wraps all expressions in a giant do-block with an IO monad. It's practically useless without the meta-language of the colon commands. That said, truly modern versions of GHC have deferred type checking for REPL use, which is impressive, useful and, ultimately, a simple emulation of a dynamic language.

Never mind that not even Python or Ruby, and especially not JavaScript, provide sensible useful REPLs with sane code reloading semantics.

People who shit on crappy type systems turn around and then shit on crappy REPLs in broken dynamic languages. I'd take a statically typed language over a language with a bad REPL any day.

> You can have a REPL in a statically typed language too---Haskell has one.

And so does Scala...

I tried Go after a few years of Ruby, and compile-time errors is what annoyed me the most. Bzzt that variable is defined but not in use, fail. Bzzt that function is defined but not used, fail. Bzzt that library is included but not used, fail. Felt like I was getting a rap across the knuckles with each failure as I was trying to learn Go.

The try/fail/modify/repeat cycle of REPL-driven-development is what appeals to me. I "get" the value of strongly typed languages, but I'd prefer to opt-in to strictness, rather than have it forced upon me.

Interesting, felt the same way, and then something changed, rather than feeling punished by the type checker, the type checker became my ally, teaching me about my mistakes at compile time.

Other than for quick scripts I have no interest in dynamic languages now, fully converted to the Typed side ;-)

Ruby and Scala are quite similar in some ways: terse syntax, functional, MOP capable (implicits), mixins (traits), etc., but yes, you have to prove to the type checker that your intentions are correct/sound.

Haskell will give you REPL-driven development and strong typechecking (an optional one in the last haskell compiler).

BTW, Go is not that strongly typed. It seems Google engineers don't like strongly typed languages.

>In some sense this is all a moot point, because any difference in development speed between statically-typed and dynamically-typed languages is dwarfed by one's familiarity with the languages in question

I dunno, I have 11 years of python experience, and almost a year of haskell experience. I am more productive in python when doing small (<100 lines) scripting type tasks. But I am noticeably more productive in haskell when writing bigger things. Haskell has a relatively crappy web ecosystem, yet I'm still more productive with snap+postgresql-simple+digestive-functors then I have ever been with any python framework (from zope way back in the day to django and flask). Tasks that are more skewed to haskell's strengths like highly concurrent network servers make the difference even more pronounced.

So yes, they would be runtime errors in Python. But that's not such a catastrophe, actually. The way I write Python is by being in an IPython shell, writing short functions, unit-testing them as I go. So the development-time cost of those runtime errors is not very high, and in my experience offset by the flexibility and interactiveness. Writing in a functional style without much mutable state is really the thing that I find saves development/debugging time.

> The way I write Python is by being in an IPython shell, writing short functions, unit-testing them as I go.

One can write Scala the same way in a Scala shell.