Merge pull request #101 from purescript/with-index-result

Let findWithIndex also return index of found value

Author: garyb

src/Data/FoldableWithIndex.purs

@@ -268,11 +268,16 @@ findWithIndex
    . FoldableWithIndex i f
   => (i -> a -> Boolean)
   -> f a
-  -> Maybe a
+  -> Maybe { index :: i, value :: a }
 findWithIndex p = foldlWithIndex go Nothing
   where
-  go i Nothing x | p i x = Just x
-  go i r _ = r
+    go
+      :: i
+      -> Maybe { index :: i, value :: a }
+      -> a
+      -> Maybe { index :: i, value :: a }
+    go i Nothing x | p i x = Just { index: i, value: x }
+    go _ r _ = r
 
 -- | A default implementation of `foldr` using `foldrWithIndex`
 foldrDefault

test/Main.purs

@@ -120,7 +120,10 @@ main = do
   assert $ find (\x -> x `mod` 2 == 0) [1, 4, 10] == Just 4
 
   log "Test findWithIndex"
-  assert $ findWithIndex (\i x -> i == 2 && x `mod` 2 == 0) [1, 2, 4, 6] == Just 4
+  assert $
+    case findWithIndex (\i x -> i `mod` 2 == 0 && x `mod` 2 == 0) [1, 2, 4, 6] of
+      Nothing -> false
+      Just { index, value } -> index == 2 && value == 4
 
   log "Test findMap" *> do
     let pred x = if x > 5 then Just (x * 100) else Nothing