（1）
Haskell requires type names to start with an uppercase letter, and variable names must start with a lowercase letter.
举例如下：
> let C = "foo"
<interactive>:20:5: Not in scope: data constructor ‘C’
> let c = "foo"
> c
"foo"
> :type c
c :: [Char]（2）
The combination of :: and the type after it is called a type signature.
举例如下：
> :type 'a'
'a' :: Char
:: Char就被称之为type signature。
（1）
The empty string is written “”, and is a synonym for [].
举例如下：
> "" == []
True
（2）
Since a string is a list of characters, we can use the regular list operators to construct new strings.
举例如下：
> 'a':"bc"
"abc"
> "foo" ++ "bar"
"foobar"
（1）
ghci stores the result of the last expression into a variable whose name is “it”. (This isn’t a Haskell language feature; it’s specific to ghci alone.)
举例如下：
> "foo"
"foo"
> it ++ "bar"
"foobar"
> it
"foobar"（2）可以使用:set <option>和:unset <option>来设置和取消一些选项。以打印计算的结果类型为例：
> :set +t
> fst ('a', 'b')
'a'
it :: Char
> :unset +t
> fst ('a', 'b')
'a'（3）在ghci中定义变量与源文件中定义变量不一样，需要在变量前加上let：
ghci> let a = 10
ghci> b = 100
<interactive>:9:3: parse error on input ‘=’