类型不明确的变量'a0'是由于使用"it' [英] Ambiguous type variable `a0' arising from a use of `it'
问题描述
我具有以下功能以返回给定数字的因子对
I have the following function to return the Factor Pairs for a given number
factorPairs:: (RealFrac a, Floating a, Integral a) => a -> [(a, a)]
factorPairs n = map(\x -> (x, div n x)) [y | y <- [1..(ceiling $ sqrt n)], n `rem` y == 0]
当我在ghci factorPairs 18
中调用该函数时,我遇到了
When I call the function in ghci factorPairs 18
I'm getting a run time error of
* Ambiguous type variable `a0' arising from a use of `it'
prevents the constraint `(Floating a0)' from being solved.
Probable fix: use a type annotation to specify what `a0' should be.
These potential instances exist:
instance Floating Double -- Defined in `GHC.Float'
instance Floating Float -- Defined in `GHC.Float'
* In the first argument of `print', namely `it'
In a stmt of an interactive GHCi command: print it
我可以用ghci对该函数进行硬编码
I can hard code the function in ghci
map(\ x->(x,div 18 x))[y |y<-[[..(ceiling $ sqrt 18)],18`rem` y == 0]
并且没有任何问题,但我似乎无法弄清楚为什么我的功能失败了.我相信ghci试图告诉我它无法弄清楚使用哪种类型来调用 print
,但是正在努力寻找解决方案.
map(\x -> (x, div 18 x)) [y | y <- [1..(ceiling $ sqrt 18)], 18 `rem` y == 0]
and don't have any issues but I can't seem to figure out why my function is failing. I believe ghci is trying to tell me it can't figure out what type to call print
with but am struggling to find the solution.
推荐答案
这与在Haskell中数字文字重载有关.当您键入 map(\ x->(x,div 18 x))[y |y<-[1 ..(ceiling $ sqrt 18)],将18`rem` y == 0]
转换为 ghci
,即 18
sqrt
的参数默认为 Double
,其他参数为 Integer
s.
This has to do with the fact numeric literals are overloaded in Haskell. When you type map(\x -> (x, div 18 x)) [y | y <- [1..(ceiling $ sqrt 18)], 18 `rem` y == 0]
into ghci
, the 18
that is an argument to sqrt
defaults to a Double
and the others to Integer
s.
但是,当你写
factorPairs:: (RealFrac a, Floating a, Integral a) => a -> [(a, a)]
factorPairs n = map(\x -> (x, div n x)) [y | y <- [1..(ceiling $ sqrt n)], n `rem` y == 0]
您强制所有 n
实例都只有一种类型.然后,问题就来了,就是根本没有满足所有这些约束的默认数字类型(实际上,我认为通常是数字类型),因此GHC会告诉您它尝试的可能的实例".解决方案是添加 fromIntegral
并放宽约束:
you force all instances of n
to have only one type. Then, the problem becomes that there simply are no default number types (in fact number types in general I think) that satisfy all of these constraints, hence GHC telling you about "possible instances" it tries. The solution is to add fromIntegral
and loosen the constraints:
factorPairs:: Integral a => a -> [(a, a)]
factorPairs n = map(\x -> (x, div n x)) [y | y <- [1..(ceiling $ sqrt $ fromIntegral n)], n `rem` y == 0]
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