这是代码:
class Problem p where
readProblem :: String -> p
solveProblem :: p -> String
readAndSolve = solveProblem . readProblem
这是 GHC 生成的错误消息:
Ambiguous type variable `b0' in the constraint:
(Problem b0) arising from a use of `readProblem'
Probable fix: add a type signature that fixes these type variable(s)
In the second argument of `(.)', namely `readProblem'
In the expression: solveProblem . readProblem
In an equation for `readAndSolve':
readAndSolve = solveProblem . readProblem
据我了解,我必须以某种方式告诉编译器,solveProblem
和readProblem
使用的Problem
实例是同一类型,但我认为没有办法声明这一点。为什么它不能自己弄清楚呢?
你不需要告诉编译器它必须是相同的类型,编译器自己计算出来。但是,它无法确定要使用哪种类型。该问题的规范著名示例是
foo = show . read
如果foo
有法律类型,那就是
foo :: (Read a, Show a) => String -> String
现在,编译器如何确定?
您的readAndSolve
将具有以下类型
readAndSolve :: Problem p => String -> String