为什么使用 PatternSynonyms 会触发非详尽匹配警告？

我正在跟进这个答案 https://stackoverflow.com/a/31106916/12162258学习如何进行模式匹配Sequence https://hackage.haskell.org/package/containers-0.6.6/docs/Data-Sequence.htmls。为了具体起见，想象一下我正在实施广度优先搜索 https://en.wikipedia.org/wiki/Breadth-first_search在二维网格上使用Sequence作为队列。仅使用ViewPatterns https://ghc.haskell.org/trac/ghc/wiki/ViewPatterns，我可能会想出类似以下的东西：

{-# 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...

下列的@仙人掌的回答 https://stackoverflow.com/a/31106916/12162258，如果我也想使用PatternSynonyms https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms，我想出：

{-# 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 = ...

这些看起来对我来说是等价的，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
            ...

我错过了什么破坏了这两个公式之间的等价性，我该如何修复它？

查看有关 COMPLETE pragmas 的 wiki 页面 https://gitlab.haskell.org/ghc/ghc/-/wikis/pattern-synonyms/complete-sigs。我将引用开头：“详尽检查器目前对模式同义词感到窒息。 它们被标记为总是容易出错的模式，这意味着我们还必须始终包含包罗万象的情况以避免警告。”

简而言之，您需要提供完整的编译指示，例如：

{-# COMPLETE Empty, (:>) #-}