如何统一c和GHC.Exts.Itemt c？

Question

我刚刚更新了我的 dynamic time warping library 以使用 ghc-7.10 编译并偶然发现了一个奇怪的问题：

在ghc-7.8中编译好的代码：

{-# LANGUAGE OverloadedLists #-}
{-# LANGUAGE TypeFamilies #-}
dtwNaive :: (Ord c, Fractional c, DataSet a, DataSet b)
         => (Item a -> Item b -> c) -> a -> b -> c
dtwNaive δ as bs = go (len as - 1) (len bs - 1)
    where go 0 0 = 0
          go _ 0 = 1/0
          go 0 _ = 1/0
          go x y = δ (ix as x) (ix bs y) + minimum [ go (x-1)  y
                                                   , go  x    (y-1)
                                                   , go (x-1) (y-1)
                                                   ]

现在产生以下错误：

src/Data/DTW.hs:112:52:
    Couldn't match type ‘c’ with ‘GHC.Exts.Item (t1 c)’
      ‘c’ is a rigid type variable bound by
          the type signature for
            dtwNaive :: (Ord c, Fractional c, DataSet a, DataSet b) =>
                        (Item a -> Item b -> c) -> a -> b -> c
          at src/Data/DTW.hs:106:13
    Expected type: Int -> [c] -> t1 c
      Actual type: Int -> [GHC.Exts.Item (t1 c)] -> t1 c
    Relevant bindings include
      go :: Int -> Int -> c (bound at src/Data/DTW.hs:109:11)
      δ :: Item a -> Item b -> c (bound at src/Data/DTW.hs:108:10)
      dtwNaive :: (Item a -> Item b -> c) -> a -> b -> c
        (bound at src/Data/DTW.hs:108:1)
    In the first argument of ‘minimum’, namely
      ‘[go (x - 1) y, go x (y - 1), go (x - 1) (y - 1)]’
    In the second argument of ‘(+)’, namely
      ‘minimum [go (x - 1) y, go x (y - 1), go (x - 1) (y - 1)]’
    In the expression:
      δ (ix as x) (ix bs y)
      + minimum [go (x - 1) y, go x (y - 1), go (x - 1) (y - 1)]

老实说，我很困惑，因为我认为 [c] 与 t ~ [] 时的 [GHC.Exts.Item (t c)] 是一回事。我错过了什么？

Answer 1

minimum的类型跟以前不一样了。旧类型是 Ord a => [a] -> a。新类型是 (Ord a, Foldable t) => t a -> a。

作为 minimum 的参数的列表文字有一个 IsList 实例不再明显。您可以通过添加本地注释来解决此问题：

{-# LANGUAGE ScopedTypeVariables #-}
dtwNaive :: 
            forall a b c.
            (Ord c, Fractional c, DataSet a, DataSet b)
         => (Item a -> Item b -> c) -> a -> b -> c
dtwNaive = ...
    minimum ([go (x-1) y, go x (y-1), go (x-1) (y-1)] :: [c])

如何统一c和GHC.Exts.Itemt c？

How to unify c and GHC.Exts.Item t c?

haskell

ghc