简而言之，我想在Haskell中进行类型检查：

both f (x, y) = (f x, f y)

foo :: ([Int], [Char])
foo = ([1], "a")

bar :: ([Int], [Char])
bar = both (concat . replicate 3) foo  -- intended: ([1, 1, 1], "aaa")

字符串
我得到的错误是：

• Couldn't match type ‘Char’ with ‘Int’
  Expected: ([Int], [Int])
    Actual: ([Int], [Char])
• In the second argument of ‘both’, namely ‘foo’
  In the expression: both (concat . replicate 3) foo
  In an equation for ‘bar’: bar = both (concat . replicate 3) foo

型
我在both中添加了以下类型注解后编译了它：

both :: (forall x. [x] -> [x]) -> ([a], [b]) -> ([a], [b])
both f (x, y) = (f x, f y)

型
但是这个解决方案并不完全令人满意，因为both看起来更通用，例如both id (1, "a")也被接受。