此post和this等其他来源坚持认为$ -operator的定义是
($) :: (a -> b) -> a -> b
f $ x = f x
或
($) f x = f x
或
($) = id
但我不明白为什么这个定义能够替换括号,所以我试图通过定义来自己重现并检查行为:
k :: (a -> b) -> a -> b
k f x = f x
但我得到的是:
-- these work:
(+2) `k` 4
(+2) `id` 4
sum `k` [1,2]
sum `id` [1,2]
map (flip(-)3) $ filter even `k` filter (>=0) [-5..10]
map (flip(-)3) $ filter even `id` filter (>=0) [-5..10]
-- these don't:
sum `k` 1:[2]
sum `id` 1:[2]
map (flip(-)3) `id` filter even $ filter (>=0) [-5..10]
不应该k
替代$
吗?我做错了什么?
答案 0 :(得分:9)
您错过了fixity declaration:
import java.util.Iterator;
import java.util.Locale;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.concurrent.atomic.AtomicLong;
import java.util.function.Function;
import java.util.function.LongConsumer;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;
public class StreamProgress
{
public static void main(String[] args)
{
int size = 250;
Stream<Integer> stream = readData(size);
LongConsumer progressConsumer = progress ->
{
// "Filter" the output here: Report only every 10th element
if (progress % 10 == 0)
{
double relative = (double) progress / (size - 1);
double percent = relative * 100;
System.out.printf(Locale.ENGLISH,
"Progress %8d, relative %2.5f, percent %3.2f\n",
progress, relative, percent);
}
};
Integer result = stream
.map(element -> process(element))
.map(progressMapper(progressConsumer))
.reduce(0, (a, b) -> a + b);
System.out.println("result " + result);
}
private static <T> Function<T, T> progressMapper(
LongConsumer progressConsumer)
{
AtomicLong counter = new AtomicLong(0);
return t ->
{
long n = counter.getAndIncrement();
progressConsumer.accept(n);
return t;
};
}
private static Integer process(Integer element)
{
return element * 2;
}
private static Stream<Integer> readData(int size)
{
Iterator<Integer> iterator = new Iterator<Integer>()
{
int n = 0;
@Override
public Integer next()
{
try
{
Thread.sleep(10);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
return n++;
}
@Override
public boolean hasNext()
{
return n < size;
}
};
return StreamSupport.stream(
Spliterators.spliteratorUnknownSize(
iterator, Spliterator.ORDERED), false);
}
}
或者在你的例子中:
infixr 0 $
Fixity声明告诉解析器中缀运算符的优先级/关联性是什么。