[英]Eta reduction for data modification
I get a warning to eta reduce the following lambda expression. 我收到一个警告,要求减少以下lambda表达式。
\(DataType arg1 arg2) -> DataType (modify arg1) arg2
The internet tells me eta-reducing means to leave out unnecessary lambdas. 互联网告诉我eta-reducing意味着省去不必要的lambda。
map (\x -> fun x) list
map fun list
How is that applicable to the code above? 这怎么适用于上面的代码? Am I perhaps just missing a basic syntax for modifying data types? 我可能只是缺少修改数据类型的基本语法?
For this case you can't eta reduce it any further. 对于这种情况,你不能再进一步减少它。 There is only a single argument to the lambda, namely (DataType arg1 arg2)
. lambda只有一个参数,即(DataType arg1 arg2)
。 These are not separate arguments, as indicated by the parentheses, but instead you are pattern matching on a constructor. 这些不是单独的参数,如括号所示,而是在构造函数上进行模式匹配。 In reality, the compiler will reduce this expression to something more like 实际上,编译器会将此表达式减少为更类似的表达式
\arg -> case arg of
DataType arg1 arg2 -> DataType (modify arg1) arg2
Here it is more clear that there is only one argument to the lambda, and it is not used in such a way that it can be reduced. 这里更清楚的是,lambda只有一个参数,并且它没有以可以减少它的方式使用。 The main rule for reduction is when you have something like 减少的主要规则是你有类似的东西
\a b c -> f b a c
The c
argument appears as the last term in the argument declaration and in the expression, so it can be dropped: c
参数显示为参数声明和表达式中的最后一个术语,因此可以删除它:
\a b -> f b a
Now, if we wanted to reduce this lambda we could define a higher order function 现在,如果我们想减少这个lambda,我们可以定义一个更高阶的函数
flip f a b = f b a
And write it as simply 并简单地写出来
flip f
(Note that flip
already exists in Prelude
). (需要注意的是flip
已经存在Prelude
)。
Another rule you can apply is when dealing with the $
operator. 您可以应用的另一个规则是处理$
运算符时。 Put simply, if you have something like 简而言之,如果你有类似的东西
\a b c -> f a $ g b $ h c
Then you can first turn this into a composition: 然后你可以先把它变成一个组合:
\a b c -> (f a . g b . h) c
And now it is simple to drop the c
argument 现在删除c
参数很简单
\a b -> f a . g b . h
And in general, you can usually swap $
s for .
一般来说,你通常可以交换$
s .
s and drop the last argument. s并删除最后一个参数。
Again, this doesn't apply in your case because your function only has 1 argument, and that argument is pattern matched on to pull out a constructor's arguments, then an entirely new value is reconstructed from it. 同样,这不适用于您的情况,因为您的函数只有1个参数,并且该参数是模式匹配的,以提取构造函数的参数,然后从中重构一个全新的值。
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