二郎 - 圖案的元組

Question

匹配。如果我寫的只是：

[X || X <- [1,2,3,4,5,6,7,8,9,10]]. 

返回：

[1,2,3,4,5,6,7,8,9,10] 

如果我寫：

[{a,b} || {a,b} <- [{1,2},{2,3}]]. 

返回

[] 

簡單的問題 - 爲什麼？

Answer 1

發電機{a，b} < - [{1,2}，{2,3}]是一個過濾器在同一時間。所以，當元素不匹配的模式，它只是跳過。

Erlang/OTP 19 [erts-8.2] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false] [dtrace] 

Eshell V8.2 (abort with ^G) 
1> [X || X <- [1,2,3,4,5,6,7,8,9,10]]. 
[1,2,3,4,5,6,7,8,9,10] 
2> [{a,b} || {a,b} <- [{1,2},{2,3}]]. 
[] 
3> [{a,b} || {a,b} <- [{1,2},{2,3},{a,b}]]. 
[{a,b}] 
4> 

你可以從learnyousomeerlang

7> Weather = [{toronto, rain}, {montreal, storms}, {london, fog},{paris, sun}, {boston, fog}, {vancouver, snow}]. 
[{toronto,rain}, 
{montreal,storms}, 
{london,fog}, 
{paris,sun}, 
{boston,fog}, 
{vancouver,snow}] 
8> FoggyPlaces = [X || {X, fog} <- Weather]. 
[london,boston] 
9> FoggyPlaces = [X || {X, fog1} <- Weather]. 
** exception error: no match of right hand side value [] 
10> [X || {X, fog1} <- Weather]. 
[] 
11> 

Answer 2

閱讀更多關於列表理解如果我寫：

[{A,B} || {A,B} <- [{1,2},{2,3}]]. 

返回

[{1,2},{2,3}]