Fix adjust modifying the wrong index. fixes #19

src/Data/Sequence.purs

@@ -270,7 +270,7 @@ unsafeAdjust f i (Seq xs) =
         g :: Elem a -> Elem a
         g = unsafeCoerce f
 
-        l' = FT.cons (g x) (force l)
+        l' = FT.snoc (force l) (g x)
       in
         Seq (FT.append l' (force r))
 

test/Data/Sequence.purs

@@ -106,6 +106,14 @@ sequenceTests = do
         result = sum (S.adjust (+1) idx' seq')
     in  result == 1 <?> "seq': " <> show seq' <> ", result: " <> show result
 
+  log "Test adjust modifies at the correct index"
+  quickCheck $ \(ArbSeq seq) idx ->
+    let seq' = const "hello" <$> S.cons 0 seq
+        idx' = integerBetween 0 (S.length seq') idx
+        result = S.index idx' (S.adjust (<> ", world") idx' seq')
+    in  (result == Just "hello, world")
+          <?> "seq': " <> show seq' <> ", result: " <> show result
+
   log "Test take"
   -- We must account for potentially negative indices generated by QuickCheck
   -- take is defined over negative indices, though the length of the result