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Diffstat (limited to 'lec07/JoinList.hs')
| -rw-r--r-- | lec07/JoinList.hs | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/lec07/JoinList.hs b/lec07/JoinList.hs index d04a0c6..7ec30a9 100644 --- a/lec07/JoinList.hs +++ b/lec07/JoinList.hs @@ -15,3 +15,16 @@ tag Append m _ _ = m (+++) :: Monoid m => JoinList m a -> JoinList m a -> JoinList m a (+++) jl1 jl2 = mappend jl1 jl2 +-- Exercise 2 + +indexJ :: (Sized b, Monoid b) => Int -> JoinList b a -> Maybe a +indexJ _ Empty = Nothing +indexJ n (Single _ a) + | n == 0 = Just a + | otherwise = Nothing +indexJ n (Append m jl1 jl2) + | n < 0 || n >= m = Nothing + | n < tag1 = indexJ n jl1 + | n >= tag1 = indexJ (n - tag1) jl2 + where tag1 = tag jl1 + |