'let' and variables in Haskell - haskell

Question

I'm a newbie in Haskell and I'm reading "Learn You a Haskell for Great Good!".
An expression that defines with a "name" and a "space", followed by "parameters" could be used as functions.
Consider the following code:
doubleMe = 2
this code actually follows the rule above, so we could see it as a function. but it really seems like an assign operations in java or c++.
So, How to understand the variable in Haskell?

Answer 1

At the top level of a program, name = expression is a definition. It creates a variable, in the sense of a mathematical variable—just a name for some expression. It can be a definition of a value:
two :: Int
two = 2
Or a definition of a function:
twice :: Int -> Int
twice x = x * two
A definition refers to a function if its type has a function arrow ->.
In a do block or GHCi, let name = expression is a local definition.
main :: IO ()
main = do
let greet name = "Hello, " ++ name ++ "!"
putStrLn (greet "world")
Finally, there is another use of the let keyword: let name = expression1 in expression2. This creates a variable local to a single expression:
length (let x = "hello" in x ++ x) == 10
Be aware that let takes a block of bindings, so it’s subject to the layout rules like other layout keywords, such as do, where, of as in case…of, and so on:
main :: IO ()
main = do
-- Bindings must be aligned past the start column.
-- ↓
let greeting name = "Hello, " ++ name ++ "!"
parting name = "Goodbye, " ++ name ++ "!"
putStrLn (greeting "world")
putStrLn (parting "world")
 
length $ let x = "hello"
y = "goodbye"
in x ++ y
As with other layout keywords, can instead put a newline and a fixed amount of indentation after let, and then not worry about alignment:
main = do -- ← newline+indent
let -- ← newline+indent
greeting name = …
parting name = …
…
 
length $ let
x = "hello"
y = "goodbye"
in x ++ y
Or you can always include explicit curly braces and semicolons:
main :: IO ();
main = do {
let {
greeting name = "Hello, " ++ name ++ "!";
parting name = "Goodbye, " ++ name ++ "!";
};
putStrLn (greeting "world");
putStrLn (parting "world");
};
length $ let {
x = "hello";
y = "goodbye";
} in x ++ y