Erlang学习

Invariable Variables

Doing arithmetic is alright, but you won't go far without being able to store results somewhere. For that, we'll use variables. If you have read the intro to this book, you'll know that variables can't be variable in functional programming. The basic behavior of variables can be demonstrated with these 7 expressions (note that variables begin with an uppercase letter):

1> One.
* 1: variable 'One' is unbound
2> One = 1.
1
3> Un = Uno = One = 1.
1
4> Two = One + One.
2
5> Two = 2.
2
6> Two = Two + 1.
** exception error: no match of right hand side value 3
7> two = 2.
** exception error: no match of right hand side value 2

Atoms

Boolean Algebra & Comparison operators

1> true and false.

false

2> false or true.

true

3> true xor false.

true

4> not false.

true

5> not (true and true).

false


6> 5 =:= 5.

true

7> 1 =:= 0.

false

8> 1 =/= 0.

true

9> 5 =:= 5.0.

false

10> 5 == 5.0.

true

11> 5 /= 5.0.

false

12> 1 < 2.

true

13> 1 < 1.

false

14> 1 >= 1.

true

15> 1 =< 1.

true

Tuples 元组

Lists 列表

Lists are the bread and butter of many functional languages. They're used to solve all kinds of problems and are undoubtedly the most used data structure in Erlang. Lists can contain anything! Numbers, atoms, tuples, other lists; your wildest dreams in a single structure. The basic notation of a list is [Element1, Element2, ..., ElementN] and you can mix more than one type of data in it:

To glue lists together, we use the ++ operator. The opposite of ++ is -- and will remove elements from a list:

5> [1,2,3] ++ [4,5].
[1,2,3,4,5]
6> [1,2,3,4,5] -- [1,2,3].
[4,5]
7> [2,4,2] -- [2,4].
[2]
8> [2,4,2] -- [2,4,2].
[]

Both ++ and -- are right-associative. This means the elements of many -- or ++ operations will be done from right to left, as in the following examples:

9> [1,2,3] -- [1,2] -- [3].
[3]
10> [1,2,3] -- [1,2] -- [2].
[2,3]

Let's keep going. The first element of a list is named the Head, and the rest of the list is named the Tail. We will use two built-in functions (BIF) to get them.

11> hd([1,2,3,4]).
1
12> tl([1,2,3,4]).
[2,3,4]

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Modules