[英]Why is Haskell throwing a 'cannot construct infinite type' error?
I wrote the following code in Haskell to compute the dot product of two vectors, but cannot compile it due to the following error: 我在Haskell中编写了以下代码来计算两个向量的点积,但由于以下错误而无法编译它:
cannot construct infinite type: a = [a] When generalising the type(s) for dot'
dot :: (Num a) => [a] -> [a] -> a
[] `dot` [] = 0
x@[xi,xs] `dot` y@[yi,ys] = xi*yi + (xs `dot` ys)
I've taken a look at this question in advance for guidance. 我事先看过这个问题以获得指导。 As far as I can tell, the types are correct.
据我所知,类型是正确的。 x, y and the two []'s are lists, and the function returns a number.
x,y和两个[]是列表,函数返回一个数字。
What's wrong? 怎么了?
Ganesh' answer is spot on. Ganesh的回答很明显。 Let me briefly elaborate on the meaning of an "infinite type".
让我简要阐述一下“无限型”的含义。
dot
has this type definition: dot
有这种类型定义:
dot :: (Num a) => [a] -> [a] -> a
This means that dot
takes two lists of Num
elements and returns a Num
. 这意味着
dot
需要两个Num
元素列表并返回Num
。 Your definition includes this line: 您的定义包括以下行:
x@[xi,xs] `dot` y@[yi,ys] = xi*yi + (xs `dot` ys)
Since, as Ganesh points out, [xi,xs]
is a list consisting of two elements, xi
and xs
should be Num
s. 因为,正如Ganesh指出的那样,
[xi,xs]
是由两个元素组成的列表, xi
和xs
应该是Num
s。 Same for yi
and ys
. 对于
yi
和ys
。 But then they are passed as arguments to dot
: 但随后它们作为参数传递给
dot
:
xs `dot` ys
This means that xs
and ys
must be lists of Num
s. 这意味着
xs
和ys
必须是Num
s的列表。 That leads to a contradiction. 这导致了矛盾。
Another way to look at this, is to for a moment forget about the type definition of dot
. 另一种看待这种情况的方法是暂时忘记
dot
的类型定义。 This line, 这条线,
x@[xi,xs] `dot` y@[yi,ys] = xi*yi + (xs `dot` ys)
states that dot
takes two lists whose elements are appropriate arguments to dot
. 表示
dot
采用两个列表,其元素是dot
适当参数。 But the only way for that to make sense, is if these lists are infinitely nested . 但唯一有意义的方法是,这些列表是否是无限嵌套的 。 That is not allowed nor sensible.
这是不允许也不明智的。
You're confusing the syntax for a two element list [x, y]
with the syntax for splitting a list into the first element and the rest of the list (x:y)
. 您将两个元素列表
[x, y]
的语法与将列表拆分为第一个元素和列表的其余部分(x:y)
的语法混淆。 Try this instead: 试试这个:
dot :: (Num a) => [a] -> [a] -> a
[] `dot` [] = 0
x@(xi:xs) `dot` y@(yi:ys) = xi*yi + (xs `dot` ys)
The @
patterns are also unnecessary, btw. @
模式也是不必要的,顺便说一下。
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