小编joh*_*son的帖子

我正在努力处理Haskell中类型类中的模糊类型变量:

这段代码将我的问题归结为基础知识:

class Problem a b where
    foo :: a -> a
    bar :: b -> b
    baz :: a -> b -> (a, b)

solve :: Problem a b => a -> b -> (a, b)
solve a b = baz (foo a) (bar b)

它没有编译,说当我调用foo和bar时我没有使用的类型变量是不明确的:

    Ambiguous type variable `b0' in the constraint:
      (Problem a b0) arising from a use of `foo'
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `baz', …