[英]why does function combine require parantesis in haskell?
Prelude> -- I have 2 functions: f and g
Prelude> f x y = x + y
Prelude> g x = 2*x
Prelude> f 2 3
5
To express $ f(x, g(y))*
with x=2 and y=3 this work well:用 x=2 和 y=3 来表达
$ f(x, g(y))*
这很好用:
Prelude> f 2 (g 3)
8
why does the following return error?为什么以下返回错误?
Prelude>
Prelude> f 2 g 3
<interactive>:19:1: error:
• Non type-variable argument in the constraint: Num (a -> a)
(Use FlexibleContexts to permit this)
• When checking the inferred type
it :: forall a. (Num a, Num (a -> a)) => a
Prelude>
f 2 g 3
is (because function application left-associative ):是(因为 function 应用程序左关联):
f 2 g 3 = ((f 2) g) 3
that's why you get this error - it expects g
there to be a Num
(as it is the parameter y
in fxy = x+y
and +:: Num a -> a -> a -> a
)这就是为什么你得到这个错误 - 它期望
g
有一个Num
(因为它是fxy = x+y
和+:: Num a -> a -> a -> a
中的参数y
)
2
as a literal can be a value in every Num a
but GHC does not know a instance for Num
that is a function a -> a
. 2
作为文字可以是每个Num a
中的值,但 GHC 不知道Num
的实例是 function a -> a
。
Now the error itself talks about the context - basic Haskell cannot have a constraints of the form Num ((->) aa)
- but you could easily (and safely) circumvent this with the given extension... then you should get the error with the type-class.现在错误本身谈到了上下文 - 基本 Haskell 不能有
Num ((->) aa)
形式的约束 - 但您可以使用给定的扩展名轻松(并且安全地)规避它......然后你应该得到错误与类型类。
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