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src/Data/List/Local.hs

@@ -1,6 +1,6 @@
 {- This file is part of Vervis.
  -
- - Written in 2016 by fr33domlover <fr33domlover@riseup.net>.
+ - Written in 2016, 2018 by fr33domlover <fr33domlover@riseup.net>.
  -
  - ♡ Copying is an act of love. Please copy, reuse and share.
  -
@@ -15,11 +15,15 @@
 
 module Data.List.Local
     ( -- groupByFst
+      groupJusts
+    , groupEithers
     )
 where
 
 import Prelude
 
+import Data.List.NonEmpty (NonEmpty (..), (<|))
+
 -- | Takes a list of pairs and groups them by consecutive ranges with equal
 -- first element. Returns a list of pairs, where each pair corresponds to one
 -- such range.
@@ -28,3 +32,25 @@ groupByFst []          = []
 groupByFst ((x, y):ps) =
     let (same, rest) = span ((== x) . fst) ps
     in  (x, y : map snd same) : groupByFst rest
+
+-- | Group together sublists of Just items, and drop the Nothing items.
+--
+-- >>> groupJusts [Nothing, Nothing, Just 1, Just 4, Nothing, Just 2]
+-- [[1, 4], [2]]
+groupJusts :: [Maybe a] -> [NonEmpty a]
+groupJusts maybes = prepend $ foldr go (Nothing, []) maybes
+    where
+    prepend (Nothing, l) = l
+    prepend (Just x , l) = x : l
+    go Nothing  (Nothing, ls) = (Nothing       , ls)
+    go Nothing  (Just l , ls) = (Nothing       , l : ls)
+    go (Just x) (Nothing, ls) = (Just $ x :| [], ls)
+    go (Just x) (Just l , ls) = (Just $ x <| l , ls)
+
+groupEithers :: [Either a b] -> ([b], [(NonEmpty a, NonEmpty b)], [a])
+groupEithers = foldr go ([], [], [])
+    where
+    go (Left x) ([]  , []         , as) = ([], []                     , x : as)
+    go (Left x) ([]  , (xs, ys):ps, as) = ([], (x <| xs, ys) : ps     , as)
+    go (Left x) (b:bs, ps         , as) = ([], (x :| [], b :| bs) : ps, as)
+    go (Right y) (bs, ps, as)           = (y : bs, ps, as)