Question

accumulate :: (a -> a) -> a -> (a -> a) -> a -> (a -> a) -> a
accumulate combiner null_value term x next n = iter x null_value
where iter x result = if x > n
                        then result
                        else iter (next x) (combiner (term x) result)

这在没有类型签名的情况下运行完美，但是使用类型签名，我一直遇到这个错误：

Couldn't match expected type `a' with actual type `a -> a'
  `a' is a rigid type variable bound by
      the type signature for
        accumulate :: (a -> a) -> a -> (a -> a) -> a -> (a -> a) -> a
      at haskell-sicp/chapter1.hs:131:15
In the expression: iter x null_value
In an equation for `accumulate':
    accumulate combiner null_value term x next n
      = iter x null_value
      where
          iter x result
            = if x > n then
                  result
              else
                  iter (next x) (combiner (term x) result)

我是haskell的新手，但我不明白我的签名有什么问题？

Answer 1

我可以告诉您，combiner方法需要两个a并返回一个a，因此它应为(a -> a -> a)，{{1需要标记为a，并且您忘记表示该函数返回Ord。所以，你的实际类型签名应该是

Haskell代码中的预期类型错误

1 个答案: