Question

我是Elixir的新手，所以我对这三个陈述感到困惑

'uname' is not recognized as an internal or external command,
operable program or batch file.

第一个和第二个语句按预期返回结果，但第三个引发错误

**（MatchError）右侧值不匹配：[[1,2,3]]

我想知道第三句为什么会犯错误

Answer 1

a匹配任何值。 [a]匹配包含一个元素的列表，该元素可以是任何值。 [[a]]匹配一个元素的列表，该列表包含另一个可以是任何值的元素的列表。

表达式[[1, 2, 3]]与前两个模式匹配，但与第三个模式不匹配，因为它是包含三个元素的一个列表的列表。

Answer 2

虽然@Dogbert的回答解释了为什么它对后一种情况不起作用，但以下是它如何工作的例子：

iex|1 ▶ [[a, _, _]] = [ [ 1, 2, 3 ] ]
#⇒ [[1, 2, 3]]
iex|2 ▶ [[^a, 2, 3]] = [ [ 1, 2, 3 ] ]
#⇒ [[1, 2, 3]]
iex|3 ▶ a
#⇒ 1

另请注意第2行中的Kernel.SpecialForms.^/1引脚运算符：它基本上强制已绑定的变量a 匹配而非反弹。< / p>

Answer 3

进一步考虑放大这个：

a = [[1,2,3]]

# a is [[1,2,3]]

[ a ] = [ [1,2,3] ]
^   ^   ^         ^ These match. 
#a is [1,2,3]

[ [ a ] ] = [ [ 1,2,3 ] ]
^ ^   ^ ^   ^ ^       ^ ^  These match as well. 

# You can consider that the parts that match on the left and the right of 
# the pattern match operator are effectively discarded and then elixir attempts to assign 
# the remaining portion on the right side to the remaining portion on the left side.
# Hence on the last expression it's trying to assign 1,2,3 to a.  The pattern doesn't match 
# because you have three values on the right but only one name on the left to bind to.

我希望这可能有助于澄清情况。 @Dogbert和@mudasobwa都是完全正确的;我希望这有助于使情况更加清晰。

不老药的Matcherror

3 个答案: