我学习this answer是为了学习如何在Sequence上进行模式匹配。具体来说，假设我在二维网格上使用Sequence作为队列来实现breadth-first search。仅使用ViewPatterns，我可能会得到如下所示的结果：

{-# LANGUAGE ViewPatterns #-}

import qualified Data.Sequence as Seq
import qualified Data.Set as Set

bfs :: Seq.Seq ((Int, Int), Int) -> Set.Set (Int, Int) -> Int
bfs (Seq.viewr -> Seq.EmptyR) _ = -1 -- goal not found
bfs (Seq.viewr -> (coords Seq.:> (coord@(r, c), dist))) seen = -- search plumbing...

根据@Cactus的回答，如果我也想使用PatternSynonyms，我会得出：

{-# LANGUAGE PatternSynonyms #-}

...

pattern Empty :: Seq.Seq a
pattern Empty <- (Seq.viewr -> Seq.EmptyR)

pattern (:>) :: Seq.Seq a -> a -> Seq.Seq a
pattern xs :> x <- (Seq.viewr -> xs Seq.:> x)

bfsPat :: Seq.Seq ((Int, Int), Int) -> Set.Set (Int, Int) -> Int
bfsPat Empty _ = -1
bfsPat (coords :> (coord@(r, c), dist)) seen = ...

在我看来，这两种方法是等价的，但编译器不同意：

In an equation for ‘bfsPat’:
        Patterns not matched:
            (Data.Sequence.Internal.Seq Data.Sequence.Internal.EmptyT)
            (Data.Set.Internal.Bin _ _ _ _)
            (Data.Sequence.Internal.Seq Data.Sequence.Internal.EmptyT)
            Data.Set.Internal.Tip
            (Data.Sequence.Internal.Seq (Data.Sequence.Internal.Single _))
            (Data.Set.Internal.Bin _ _ _ _)
            (Data.Sequence.Internal.Seq (Data.Sequence.Internal.Single _))
            Data.Set.Internal.Tip
            ...

我错过了什么，打破了这两个公式之间的等效性，我如何才能修复它？