如果">{-#LANGUAGE OverloadedStrings#-}";包含在源文件的顶部,或package.yaml中(我使用的是堆栈(,然后是
length "a" -- does not compile anymore.
然而,自定义功能length'
在上运行良好
length' :: [a] -> Int
length' xs = sum [1 | _ <- xs]
包Data.String
是导入的-我认为问题是存在的,但是,我很想看看是否有人有类似的问题。
堆栈和GHC版本:2.3.1版,Git修订版x86_64 hpack-0.33.0,ghc-8.8.3
我使用的是mac-osx,但同样的错误也出现在Linux和windows中。
这是堆栈产生的错误:
/Users/admin1/Haskell/PROJECTS/orig1/src/Lib.hs:13:29: error:
• Ambiguous type variables ‘t0’,
‘a0’ arising from the literal ‘"a"’
prevents the constraint ‘(IsString (t0 a0))’ from being solved.
Probable fix: use a type annotation to specify what ‘t0’,
‘a0’ should be.
These potential instances exist:
instance (a ~ Char) => IsString [a] -- Defined in ‘Data.String’
...plus two instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the first argument of ‘length’, namely ‘"a"’
In the first argument of ‘show’, namely ‘(length "a")’
In the second argument of ‘($)’, namely ‘show (length "a")’
|
13 | putStrLn $ show (length "a") -- does not work, if "- OverloadedStrings" is on
这是因为length
有一个签名length :: Foldable f => f a -> Int
,所以它可以是任何Foldable
类型。如果使用OverloadedStrings
扩展,则"foo"
不再是String
,它可以是任何类型的IsString a => a
,其中的多个也可以是Foldable f => f a
s。
你可以做的是给编译器一个类型提示,例如:
length ("a":: String)