Java requires more code than Python or Haskell because it's extremely imperative and statement based. Python on the other hand includes a lot of functional, more expressive idioms like concise list comprehensions.
Documentation is more than a function definition. Types don’t explain rationales and how to use functions and programs. Types don’t give proper examples.
Bob: Sure, Alice, what does it do?
Alice: It takes an object and it returns an object.
Bob: But what does it actually do?
Alice: I told you. It takes an object and it returns an object.
Bob: But what does it do with the object? What's it for? Why should I use it?
Alice: Hey, I just gave you the full documentation for my function. You have everything you need. Now go use it.
Bob: Um, no thanks. I like to know more about what I'm getting myself in to.
Bob: Sure, Alice, what does it do?
Alice: It takes a string and it returns an integer.
Bob: But what does it actually do?
Alice: I told you. It takes a string and it returns an integer.
Bob: But what does it do with the string? What's it for? Why should I use it?
Alice: Hey, I just gave you the full documentation for my function. You have everything you need. Now go use it.
Bob: Um, no thanks. I like to know more about what I'm getting myself in to.
By the time you've added type constrains through typeclasses, selected types named after the domain, and selected a suitable name for the function, it's possible to write against the signature with no knowledge of the body or comments.
You're also forgetting that signature includes the name of the function.
reverse :: [a] -> [a] is easy to understand by it's name and type.
> You're also forgetting that signature includes the name of the function.
That's what I wanted to point out. The name of a function is even more important than its types. Types alone seldomly give you the complete picture of what's going on in a function.
Of course the name can be wrong whereas types can't, but the name is less likely to be wrong than a comment.
In Haskell you start by defining your domain as types, and constrain those types with typeclasses. By the time you get to writing signatures, the implementations can almost be inferred (in Idris they actually can be inferred, the code literally writes itself).
In an imperative language with side effects, examples and documentation are a must because behavior is hidden in the body of a function; the signatures lie.
A journeyman Haskeller can write code that is completely self documenting. No examples are required, because the types coupled with the purity of the language allows us to tell the whole story.
I have experienced what you're talking about with Haskell but it was a lot of uncomfortable work. But it's "technically" true that the types often describe how to use a library, sometimes so well that errors cannot happen.
I don't think that's true. Idiomatic Haskell has explicit declarations only for top level functions and uses type inference for everything else.
The same can be true of tests. I've started generating API documentation from mine: e.g. https://hitchdev.com/strictyaml/using/alpha/scalar/email-and...
I tend to think of types and tests as attacking the same problem from opposite directions.
Public Service Announcement: don't validate emails with regex. Every time you do, god kills a kitten.
Maybe that's partly because Haskell can easily become a bit too elegant. Between all the currying and combinators, the type helps to understand code "top down", i.e. when you don't have studied and memorized all "bottom up" component parts.