Why is Clojure dynamically typed?

One thing I like very much is reading about different programming languages. Currently, I'm learning Scala but that doesn't mean I'm not interested in Groovy, Clojure, Python, and many others. All these languages have a unique look and feel and some characteristic features. In the case of Clojure I don't understand one of these design decisions. As far as I know, Clojure puts great emphasis on its functional paradigm and pretty much forces you to use immutable "variables" wherever possible. So if half of your values are immutable, why is the language dynamically typed?

The Clojure website says:

First and foremost, Clojure is dynamic. That means that a Clojure program is not just something you compile and run, but something with which you can interact.

Well, that sounds completely strange. If a program is compiled you can't change it anymore. Sure you can "interact" with it, that's what UIs are used for but the website certainly doesn't mean a neat "dynamic" GUI.

How does Clojure benefit from dynamical typing

I mean the special case of Clojure and not general advantages of dynamic typing.

How does the dynamic type system help improve functional programming

Again, I know the pleasure of not spilling "int a;" all over the source code but type inference can ease a lot of the pain. Therefore I would just like to know how dynamic typing supports the concepts of a functional language.

If a program is compiled you can't change it anymore.

This is wrong. In image-based systems, like Lisp (Clojure can be seen as a Lisp dialect) and Smalltalk, you can change the compiled environment. Development in such a language typically means working on a running system, adding and changing function definitions, macro definitions, parameters etc. (adding means compiling and loading into the image).

This has a lot of benefits. For one, all the tools can interact directly with the program and do not need to guess at the system's behaviour. You also do not have any long compilation pauses, because each compiled unit is very small (it is very rare to recompile everything). The NASA JPL once corrected a running Lisp system on a probe hundreds of thousands of kilometres away in space.

For such a system, it is very natural to have type information available at runtime (that is what dynamic typing means). Of course, nothing hinders you from also doing type inference and type checks at compilation time. These concepts are orthogonal. Modern Lisp implementations typically can do both.

Well first of all Clojure is a Lisp and Lisps traditionally have always been dynamically typed.

Second as the excerpt you quoted said Clojure is a dynamic language. This means, among other things, that you can define new functions at runtime, evaluate arbitrary code at runtime and so on. All of these things are hard or impossible to do in statically typed languages (without plastering casts all over the place).

Another reason is that macros might complicate debugging type errors immensely. I imagine that generating meaningful error messages for type errors produced by macro-generated code would be quite a task for the compiler.

I agree, a purely functional language can still have an interactive read-eval-print-loop, and would have an easier time with type inference. I assume Clojure wanted to attract lisp programmers by being "lisp for the jvm", and chose to be dynamic like other lisps. Another factor is that type systems need to be designed as the very first step of the language, and it's faster for language implementors to just skip that step.

(I'm rephrasing the original answer since it generated too much misunderstanding)

One of the reasons to keep Clojure (and any Lisp) dynamically typed is to simplify creation of macros. In short, macros deal with abstract syntax trees (ASTs) which can contain nodes of many, many different types (usually, any objects at all). In theory, it's possible to make full statically typed macro system, but in practice such systems are usually limited and sparsely spread. Please, see examples below and extended discussion in the thread.

EDIT 2020: Wow, 9 years passed from the time I posted this answer, and people still add comments. What a legacy we all have left!

Some people noted in comments that having a statically typed language doesn't prevent you from expressing code as data structure. And, strictly speaking, it's true - union types allow to express data structures of any complexity, including syntax of a language. However I claim that to express the syntax, you must either reduce expressiveness, or use such wide unions that you lose all advantages of static typing. To prove this claim I will use another language - Julia.

Julia is optionally typed - you can constrain any function or struct field to have a particular type, and Julia will check it. The language supports AST as a first class citizen using Expr and Symbol types. Expression definition looks something like this:

struct Expr
  head::Symbol
  args::Vector{Any}
end

Expression consists of a head which is always a symbol and list of arguments which may have any types. Julia also supports special Union which can constrain argument to specific types, e.g. Symbols and other Exprs:

struct Expr
  head::Symbol
  args::Vector{Union{Symbol, Expr}}
end

Which is sufficient to express e.g. :(x + y):

dump(:(x + y))
Expr
  head: Symbol call
  args: Array{Any}((3,))
    1: Symbol +
    2: Symbol x
    3: Symbol y

But Julia also supports a number of other types in expressions. One obvious and helpful example is literals:

:(x + 1)

Moreover, you can use interpolation or construct expressions manually to put any object to AST:

obj = create_some_object()

ex1 = :(x + $objs)
ex2 = Expr(:+, :x, obj)

These examples are not just a funny experiments, they are actively used in real code, especially in macros. So you cannot constrain expression arguments to a specific union of types - expressions may contain any values.

Of course, when designing a new language you can put any restrictions on it. Perhaps, restricting Expr to contain only Symbol, Expr and some Literals would be useful in some contexts. But it goes against principles of simplicity and flexibility in both - Julia and Clojure, and would significantly reduce usefulness of macros.