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fix empty case in 'filter', and note differences in semantics wrt. focus to 'delete'

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Don Stewart
2007-06-26 03:57:41 +00:00
parent 977f8328fc
commit ce28fc1eb2
1 changed files with 9 additions and 4 deletions
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13
StackSet.hs
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@@ -302,11 +302,16 @@ differentiate (x:xs) = Just $ Stack x [] xs
-- /O(n)/. 'filter p s' returns the elements of 's' such that 'p' evaluates to -- /O(n)/. 'filter p s' returns the elements of 's' such that 'p' evaluates to
-- True. Order is preserved, and focus moves to the next node to the right (if -- True. Order is preserved, and focus moves to the next node to the right (if
-- necessary). -- necessary).
--
-- Note, this isn't the same as the 'remove' semantics, as focus
-- won't move 'left' on the end of list.
--
filter :: (a -> Bool) -> Stack a -> StackOrNot a filter :: (a -> Bool) -> Stack a -> StackOrNot a
filter p (Stack f ls rs) = Just $ case L.filter p (f:rs) of filter p (Stack f ls rs) = case L.filter p (f:rs) of
(f':rs') -> Stack f' (L.filter p ls) rs' f':rs' -> Just $ Stack f' (L.filter p ls) rs' -- maybe move focus down
_ -> Stack f' [] rs' [] -> case L.filter p (reverse ls) of -- filter back up
where (f':rs') = reverse (L.filter p ls) f':rs' -> Just $ Stack f' [] rs' -- else up
[] -> Nothing
-- | -- |
-- /O(s)/. Extract the stack on the current workspace, as a list. -- /O(s)/. Extract the stack on the current workspace, as a list.