Implement pattern synonyms for StrictText and LazyText

src/Data/Text.hs

@@ -4,6 +4,8 @@
 {-# LANGUAGE UnliftedFFITypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE PartialTypeSignatures #-}
+{-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE ViewPatterns #-}
 
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 {-# OPTIONS_GHC -Wno-partial-type-signatures #-}
@@ -61,6 +63,11 @@ module Data.Text
     , singleton
     , empty
 
+    -- * Pattern matching
+    , pattern Empty
+    , pattern (:<)
+    , pattern (:>)
+
     -- * Basic interface
     , cons
     , snoc
@@ -565,6 +572,34 @@ null (Text _arr _off len) =
  "TEXT null/empty -> True" null empty = True
 #-}
 
+-- | Bidirectional pattern synonym for 'empty' and 'null' (both /O(1)/),
+-- to be used together with '(:<)' or '(:>)'.
+--
+-- @since 2.1.2
+pattern Empty :: Text
+pattern Empty <- (null -> True) where
+  Empty = empty
+
+-- | Bidirectional pattern synonym for 'cons' (/O(n)/) and 'uncons' (/O(1)/),
+-- to be used together with 'Empty'.
+--
+-- @since 2.1.2
+pattern (:<) :: Char -> Text -> Text
+pattern x :< xs <- (uncons -> Just (x, xs)) where
+  (:<) = cons
+infixr 5 :<
+{-# COMPLETE Empty, (:<) #-}
+
+-- | Bidirectional pattern synonym for 'snoc' (/O(n)/) and 'unsnoc' (/O(1)/)
+-- to be used together with 'Empty'.
+--
+-- @since 2.1.2
+pattern (:>) :: Text -> Char -> Text
+pattern xs :> x <- (unsnoc -> Just (xs, x)) where
+  (:>) = snoc
+infixl 5 :>
+{-# COMPLETE Empty, (:>) #-}
+
 -- | /O(1)/ Tests whether a 'Text' contains exactly one character.
 isSingleton :: Text -> Bool
 isSingleton = S.isSingleton . stream

src/Data/Text/Internal/Lazy.hs

@@ -50,7 +50,9 @@ import qualified Data.Text.Internal as T
 import qualified Data.Text as T
 
 data Text = Empty
+          -- ^ Empty text.
           | Chunk {-# UNPACK #-} !T.Text Text
+          -- ^ Chunks must be non-empty, this invariant is not checked.
             deriving (Typeable)
 
 -- | Type synonym for the lazy flavour of 'Text'.

src/Data/Text/Lazy.hs

@@ -3,6 +3,8 @@
 {-# LANGUAGE Trustworthy #-}
 {-# LANGUAGE TemplateHaskellQuotes #-}
 {-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE ViewPatterns #-}
 
 -- |
 -- Module      : Data.Text.Lazy
@@ -60,6 +62,11 @@ module Data.Text.Lazy
     , foldrChunks
     , foldlChunks
 
+    -- * Pattern matching
+    , pattern Empty
+    , pattern (:<)
+    , pattern (:>)
+
     -- * Basic interface
     , cons
     , snoc
@@ -533,6 +540,26 @@ null Empty = True
 null _     = False
 {-# INLINE [1] null #-}
 
+-- | Bidirectional pattern synonym for 'cons' (/O(n)/) and 'uncons' (/O(1)/),
+-- to be used together with 'Empty'.
+--
+-- @since 2.1.2
+pattern (:<) :: Char -> Text -> Text
+pattern x :< xs <- (uncons -> Just (x, xs)) where
+  (:<) = cons
+infixr 5 :<
+{-# COMPLETE Empty, (:<) #-}
+
+-- | Bidirectional pattern synonym for 'snoc' (/O(n)/) and 'unsnoc' (/O(1)/)
+-- to be used together with 'Empty'.
+--
+-- @since 2.1.2
+pattern (:>) :: Text -> Char -> Text
+pattern xs :> x <- (unsnoc -> Just (xs, x)) where
+  (:>) = snoc
+infixl 5 :>
+{-# COMPLETE Empty, (:>) #-}
+
 -- | /O(1)/ Tests whether a 'Text' contains exactly one character.
 isSingleton :: Text -> Bool
 isSingleton = S.isSingleton . stream