我以为我总是可以把 undefined
在我还不知道该放什么的同一个地方,并且代码应该可以正常编译,只有在 undefined
的情况下才会在运行时发生错误实际上是评估的。
然而,就在这样做时,我开始写类似的东西
f = foldl undefined undefined undefined
在文件中,当我尝试加载文件时 GHCi 会出现此错误source.hs:3:7: error:
• Ambiguous type variable ‘t0’ arising from a use of ‘foldl’
prevents the constraint ‘(Foldable t0)’ from being solved.
Probable fix: use a type annotation to specify what ‘t0’ should be.
These potential instances exist:
instance Foldable (Either a) -- Defined in ‘Data.Foldable’
instance Foldable Maybe -- Defined in ‘Data.Foldable’
instance Foldable ((,) a) -- Defined in ‘Data.Foldable’
...plus one other
...plus 29 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the expression: foldl undefined undefined undefined
In an equation for ‘f’: f = foldl undefined undefined undefined
|
3 | f = foldl undefined undefined undefined
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
我将其解释为“undefined
此处不进行类型检查”。这种行为背后的原因是什么,哪些地方不能使用
undefined
作为临时占位符?
最佳答案
解释器不知道 f
的类型在 foldl :: Foldable f => (a -> b -> a) -> a -> f b -> a
使用,因此错误。这很重要,因为 f
这里决定了 foldl
的具体实现。它会使用。确实对于 f ~ Maybe
实现可以不同于 f ~ []
,因此取决于 f
的类型,不同的foldl
用来。
如果我们指定类型,例如使用 [Int]
, 或 Maybe Char
,这有效(它会引发错误,但这是因为 undefined
当然不会正确评估):
Prelude> foldl undefined undefined (undefined :: Maybe Char)
*** Exception: Prelude.undefined
CallStack (from HasCallStack):
error, called at libraries/base/GHC/Err.hs:78:14 in base:GHC.Err
undefined, called at <interactive>:1:6 in interactive:Ghci1
Prelude> foldl undefined undefined (undefined :: [Int])
*** Exception: Prelude.undefined
CallStack (from HasCallStack):
error, called at libraries/base/GHC/Err.hs:78:14 in base:GHC.Err
undefined, called at <interactive>:2:6 in interactive:Ghci1
对于某些类型类,如 Num
,有type defaulting [Wkang's Haskell; blog] .这使用:default Num Integer default Real Integer default Enum Integer default Integral Integer default Fractional Double default RealFrac Double default Floating Double default RealFloat Double
关于haskell - 为什么我不能在任何地方都使用 undefined ?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/66892098/