问题描述
import qualified Data.Map as M
incr c a = M.insertWith (+) a 1 c
counter = foldl incr M.empty
main = print 5
当我尝试在 macOS 上使用 ghc 版本 8.8.4 进行编译时，我遇到了一大堆错误（如下）。但是，当我将每一行（通过粘贴）手动执行到 ghci 中时，没有错误。
为什么此代码在 ghci 中有效，但在我尝试编译或 :load 时无效？
Bar.hs:4:11: error:
• Ambiguous type variable ‘t0’ arising from a use of ‘foldl’
prevents the constraint ‘(Foldable t0)’ from being solved.
Relevant bindings include
counter :: t0 k0 -> M.Map k0 Integer (bound at Bar.hs:4:1)
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 (M.Map k) -- Defined in ‘Data.Map.Internal’
instance Foldable Maybe -- Defined in ‘Data.Foldable’
...plus two others
...plus 26 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the expression: foldl incr M.empty
In an equation for ‘counter’: counter = foldl incr M.empty
|
4 | counter = foldl incr M.empty
| ^^^^^^^^^^^^^^^^^^
Bar.hs:4:17: error:
• Ambiguous type variable ‘k0’ arising from a use of ‘incr’
prevents the constraint ‘(Ord k0)’ from being solved.
Relevant bindings include
counter :: t0 k0 -> M.Map k0 Integer (bound at Bar.hs:4:1)
Probable fix: use a type annotation to specify what ‘k0’ should be.
These potential instances exist:
instance (Ord a,Ord b) => Ord (Either a b)
-- Defined in ‘Data.Either’
instance (Ord k,Ord v) => Ord (M.Map k v)
-- Defined in ‘Data.Map.Internal’
instance Ord Ordering -- Defined in ‘GHC.Classes’
...plus 24 others
...plus 38 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the first argument of ‘foldl’,namely ‘incr’
In the expression: foldl incr M.empty
In an equation for ‘counter’: counter = foldl incr M.empty
|
4 | counter = foldl incr M.empty
| ^^^^
解决方法
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