assistant: There is no exact definition for what a functional programming language is, but functional languages tend to emphasize a few key features. Among them are:
- First class functions: the ability to store and manipulate functions as if they were ordinary variables.
- Pure functions: functions which have so-called "side-effects," or consequences beyond simply returning an output.
- Declarative nature: functional languages tend to be declarative, meaning that instead of a program being a list of in-order instructions, a program is more often than not a graph of dependencies and definitions.

prompter: When should I use functional programming language and an imperative language?

prompter: Why would I want to treat a function like a variable?

assistant: A simple practical example might be sorting a list based on custom criteria. For example, let's say you want to sort a list of strings alphabetically, as opposed to the usual default "ASCIIbetical" order (putting all uppercase letters above all lowercase ones). Under an imperative approach, you would have to use or define a separate sort function that uses a case-insensitive comparison. In a functional language, on the other hand, the sort function can simply accept an optional custom comparison function. Python (which has functional aspects) takes a similar approach, with `sorted()` accepting a "key" function that is called once per element, returning a new value that will be used for comparisons. Here's how we could write our case-insensitive sorting function:
```python
sorted(["Python", "is", "pretty", "COOL"], key=str.lower)
# ["COOL", "is", "pretty", "Python"]
```
Compare that to the default behavior:
```python
sorted(["Python", "is", "pretty", "COOL"])
# ["COOL", "Python", "is", "pretty"]
```
Functions can also be defined dynamically. An example in Python is decorators, which are functions that modify other functions as they're being defined. Here's an example of a decorator that logs function calls:
```python
def autolog(func):
    def wrapper(*args, **kwargs):
        print(f"Function {func.__name__} called")
        ret = func(*args, **kwargs)
        print(f"Returned: {ret}")
        return ret
    return wrapper

@autolog
def say_hi(name):
    print(f"Hi, {name}!")
    return "I just greeted someone!"

say_hi("Greta")
# Function say_hi called
# Hi, Greta!
# Returned: I just greeted someone!
```
Note that the `@` symbol is syntactic sugar for calling the decorator. The following code does the same thing:
```python
def say_hi_unwrapped(name):
    print(f"Hi, {name}!")
    return "I just greeted someone!"

say_hi = autolog(say_hi_unwrapped)
```

assistant: A functional programming language is a programming paradigm that emphasizes the use of "pure functions" - functions that have no side-effects and always return the same output for a given input. They rely on mathematical functions, recursion and expressions rather than statements and imperative commands. Functional programming languages are designed to make it easy to reason about the code and to avoid bugs that can be caused by changes in the state of the program.

In contrast, imperative programming languages are based on the idea of giving the computer a set of "imperative" commands to execute, such as assignments and loops. These languages often rely on the use of variables and mutable state, which can make it more difficult to reason about the code and can lead to bugs caused by unexpected changes in the state of the program.

Functional programming languages are different from imperative languages in the way they treat computation and data. In Functional languages, functions are first-class citizens and data is immutable, which means it cannot be changed once it's created, this makes it easier to reason about the code and to avoid bugs.

Examples of functional programming languages include Lisp, Scheme, Haskell, OCaml, and F#. Examples of imperative programming languages include C, C++, Java, Python and C#.

It's worth noting that many modern programming languages support both functional and imperative programming paradigms, so it's possible to write both functional and imperative code in the same language.