Code Review Request - Unbalanced Parenthesis correction - Haskell-Cafe

Code Review Request - Unbalanced Parenthesis correction

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Michael Litchard

21 Mar 2017 21 Mar '17

12:53 p.m.

I'm prepping for a coding interview, and am examining the task of correcting unbalanced parentheses. The finger tree seems to be the right data structure. As a proof of concept I've used Data.Sequence to test my idea. If this is the right direction to go, I'll write more specialized finger tree code. The code works on the few test cases I have tried. Feedback appreciated. {-# LANGUAGE ViewPatterns #-}module Parenthesis whereimport BasicPrelude hiding (concat,null,empty) import Data.Sequence hiding (length)import Data.Foldable hiding (length,null) balanceParens :: String -> String balanceParens str = go str [] empty where go [] [] (null -> True) = [] go [] [] parens = Data.Foldable.toList parens go ('(':xs) [] (null -> True) = go xs [RP] (singleton '(') go (')':xs) [] (null -> True) = go xs [] (fromList "()") go ('(':xs) debit parens = go xs (RP:debit) (parens |> '(') go (')':xs) [] parens = go xs [] corrected where corrected = ('(' <| parens) |> ')' go (')':xs) (RP:debit) parens = go xs debit (parens |> ')') go (_:xs) debit parens = go xs debit parens go [] (RP:debit) parens = go [] debit (parens |> ')') example: balanceParens "))(""(())()" balanceParens ")))""((()))"

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David Turner

21 Mar 21 Mar

1:13 p.m.

Hi Michael, Are there any strange constraints on the problem, such as the input string being 10s of GB long, or any particular correction strategy being required? Your code seems to add some parens to the start/end of the entire string to balance it, whereas I think I might have balanced "))(" to "()()()" given free reign. Is the goal efficiency or clarity or something else?

...

From personal taste, I would recommend including a suite of automated tests alongside your implementation even if it is not explicitly requested. A quickcheck test asserting that the output is always balanced and that the input string is a substring of the output might be appropriate.

On 21 March 2017 at 16:53, Michael Litchard wrote:

...

I'm prepping for a coding interview, and am examining the task of correcting unbalanced parentheses. The finger tree seems to be the right data structure. As a proof of concept I've used Data.Sequence to test my idea. If this is the right direction to go, I'll write more specialized finger tree code. The code works on the few test cases I have tried. Feedback appreciated.

{-# LANGUAGE ViewPatterns #-}module Parenthesis whereimport BasicPrelude hiding (concat,null,empty) import Data.Sequence hiding (length)import Data.Foldable hiding (length,null)

balanceParens :: String -> String balanceParens str = go str [] empty where go [] [] (null -> True) = [] go [] [] parens = Data.Foldable.toList parens go ('(':xs) [] (null -> True) = go xs [RP] (singleton '(') go (')':xs) [] (null -> True) = go xs [] (fromList "()") go ('(':xs) debit parens = go xs (RP:debit) (parens |> '(') go (')':xs) [] parens = go xs [] corrected where corrected = ('(' <| parens) |> ')' go (')':xs) (RP:debit) parens = go xs debit (parens |> ')') go (_:xs) debit parens = go xs debit parens go [] (RP:debit) parens = go [] debit (parens |> ')')

example:

balanceParens "))(""(())()" balanceParens ")))""((()))"

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David McBride

1:16 p.m.

Whether your algorithm is correct depends on how you are supposed to rebalance them. My naive attempt gives very different results. bal :: String -> String bal = go 0 where go :: Int -> String -> String go 0 "" = "" go n "" = replicate n ')' go n ('(':xs) = '(' : (go (n + 1) xs) go 0 (')':xs) = '(' : ')' : (go 0 xs) go n (')':xs) = ')' : (go (n - 1) xs) bal "))(" "()()()" bal ")))" "()()()" On Tue, Mar 21, 2017 at 12:53 PM, Michael Litchard wrote:

...

I'm prepping for a coding interview, and am examining the task of correcting unbalanced parentheses. The finger tree seems to be the right data structure. As a proof of concept I've used Data.Sequence to test my idea. If this is the right direction to go, I'll write more specialized finger tree code. The code works on the few test cases I have tried. Feedback appreciated.

{-# LANGUAGE ViewPatterns #-} module Parenthesis where import BasicPrelude hiding (concat,null,empty)

import Data.Sequence hiding (length) import Data.Foldable hiding (length,null)

balanceParens :: String -> String balanceParens str = go str [] empty where go [] [] (null -> True) = [] go [] [] parens = Data.Foldable.toList parens go ('(':xs) [] (null -> True) = go xs [RP] (singleton '(') go (')':xs) [] (null -> True) = go xs [] (fromList "()") go ('(':xs) debit parens = go xs (RP:debit) (parens |> '(') go (')':xs) [] parens = go xs [] corrected where corrected = ('(' <| parens) |> ')' go (')':xs) (RP:debit) parens = go xs debit (parens |> ')') go (_:xs) debit parens = go xs debit parens go [] (RP:debit) parens = go [] debit (parens |> ')')

example:

balanceParens "))(" "(())()" balanceParens ")))" "((()))"

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David Turner

1:20 p.m.

My attempt to replicate the OP's strategy but not impl: balanceParens :: String -> String balanceParens s = replicate neededOpening '(' ++ s ++ replicate neededClosing ')' where depthChange '(' = 1 depthChange ')' = -1 depthChange _ = 0 depths = scanl (+) 0 $ map depthChange s neededOpening = negate $ minimum depths neededClosing = last depths + neededOpening On 21 March 2017 at 17:16, David McBride wrote:

...

Whether your algorithm is correct depends on how you are supposed to rebalance them. My naive attempt gives very different results.

bal :: String -> String bal = go 0 where go :: Int -> String -> String go 0 "" = "" go n "" = replicate n ')' go n ('(':xs) = '(' : (go (n + 1) xs) go 0 (')':xs) = '(' : ')' : (go 0 xs) go n (')':xs) = ')' : (go (n - 1) xs)

bal "))(" "()()()"

bal ")))" "()()()"

...
I'm prepping for a coding interview, and am examining the task of correcting unbalanced parentheses. The finger tree seems to be the right data structure. As a proof of concept I've used Data.Sequence to test my idea. If this is the right direction to go, I'll write more specialized finger

On Tue, Mar 21, 2017 at 12:53 PM, Michael Litchard wrote: tree

...
code. The code works on the few test cases I have tried. Feedback appreciated.

{-# LANGUAGE ViewPatterns #-} module Parenthesis where import BasicPrelude hiding (concat,null,empty)

import Data.Sequence hiding (length) import Data.Foldable hiding (length,null)

balanceParens :: String -> String balanceParens str = go str [] empty where go [] [] (null -> True) = [] go [] [] parens = Data.Foldable.toList parens go ('(':xs) [] (null -> True) = go xs [RP] (singleton '(') go (')':xs) [] (null -> True) = go xs [] (fromList "()") go ('(':xs) debit parens = go xs (RP:debit) (parens |> '(') go (')':xs) [] parens = go xs [] corrected where corrected = ('(' <| parens) |> ')' go (')':xs) (RP:debit) parens = go xs debit (parens |> ')') go (_:xs) debit parens = go xs debit parens go [] (RP:debit) parens = go [] debit (parens |> ')')

example:

balanceParens "))(" "(())()" balanceParens ")))" "((()))"

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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Michael Litchard

1:44 p.m.

I got this idea from looking at glassdoor comments of previous interviewees. The spec was vague, but I imagined that the requirement would need to be efficient and keep contiguous parenthesis unaltered. so balanceParens ")))" == "()()()" would be incorrect. On Tue, Mar 21, 2017 at 10:20 AM, David Turner < dct25-561bs@mythic-beasts.com> wrote:

...

My attempt to replicate the OP's strategy but not impl:

balanceParens :: String -> String balanceParens s = replicate neededOpening '(' ++ s ++ replicate neededClosing ')' where depthChange '(' = 1 depthChange ')' = -1 depthChange _ = 0

depths = scanl (+) 0 $ map depthChange s neededOpening = negate $ minimum depths neededClosing = last depths + neededOpening

On 21 March 2017 at 17:16, David McBride wrote:

...
Whether your algorithm is correct depends on how you are supposed to rebalance them. My naive attempt gives very different results.

bal :: String -> String bal = go 0 where go :: Int -> String -> String go 0 "" = "" go n "" = replicate n ')' go n ('(':xs) = '(' : (go (n + 1) xs) go 0 (')':xs) = '(' : ')' : (go 0 xs) go n (')':xs) = ')' : (go (n - 1) xs)

bal "))(" "()()()"

bal ")))" "()()()"

...
I'm prepping for a coding interview, and am examining the task of correcting unbalanced parentheses. The finger tree seems to be the right data structure. As a proof of concept I've used Data.Sequence to test my idea. If this is the right direction to go, I'll write more specialized finger

On Tue, Mar 21, 2017 at 12:53 PM, Michael Litchard wrote: tree

...
code. The code works on the few test cases I have tried. Feedback appreciated.

{-# LANGUAGE ViewPatterns #-} module Parenthesis where import BasicPrelude hiding (concat,null,empty)

import Data.Sequence hiding (length) import Data.Foldable hiding (length,null)

balanceParens :: String -> String balanceParens str = go str [] empty where go [] [] (null -> True) = [] go [] [] parens = Data.Foldable.toList parens go ('(':xs) [] (null -> True) = go xs [RP] (singleton '(') go (')':xs) [] (null -> True) = go xs [] (fromList "()") go ('(':xs) debit parens = go xs (RP:debit) (parens |> '(') go (')':xs) [] parens = go xs [] corrected where corrected = ('(' <| parens) |> ')' go (')':xs) (RP:debit) parens = go xs debit (parens |> ')') go (_:xs) debit parens = go xs debit parens go [] (RP:debit) parens = go [] debit (parens |> ')')

example:

balanceParens "))(" "(())()" balanceParens ")))" "((()))"

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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David Turner

1:52 p.m.

... or it could be a ploy to see how you deal with incomplete or vague specs; how you ask clarifying questions etc. Adding sufficiently many opening parens at the start can't obviously be done without traversing the whole input first, whereas adding them as you discover excesses of closing parens is possible to implement in a streaming fashion, i.e. with O(1) memory usage and only traversing the input once. On 21 Mar 2017 17:44, "Michael Litchard" wrote: I got this idea from looking at glassdoor comments of previous interviewees. The spec was vague, but I imagined that the requirement would need to be efficient and keep contiguous parenthesis unaltered. so balanceParens ")))" == "()()()" would be incorrect. On Tue, Mar 21, 2017 at 10:20 AM, David Turner < dct25-561bs@mythic-beasts.com> wrote:

...

My attempt to replicate the OP's strategy but not impl:

balanceParens :: String -> String balanceParens s = replicate neededOpening '(' ++ s ++ replicate neededClosing ')' where depthChange '(' = 1 depthChange ')' = -1 depthChange _ = 0

depths = scanl (+) 0 $ map depthChange s neededOpening = negate $ minimum depths neededClosing = last depths + neededOpening

On 21 March 2017 at 17:16, David McBride wrote:

...
Whether your algorithm is correct depends on how you are supposed to rebalance them. My naive attempt gives very different results.

bal :: String -> String bal = go 0 where go :: Int -> String -> String go 0 "" = "" go n "" = replicate n ')' go n ('(':xs) = '(' : (go (n + 1) xs) go 0 (')':xs) = '(' : ')' : (go 0 xs) go n (')':xs) = ')' : (go (n - 1) xs)

bal "))(" "()()()"

bal ")))" "()()()"

...
I'm prepping for a coding interview, and am examining the task of correcting unbalanced parentheses. The finger tree seems to be the right data structure. As a proof of concept I've used Data.Sequence to test my idea. If this is the right direction to go, I'll write more specialized finger

On Tue, Mar 21, 2017 at 12:53 PM, Michael Litchard wrote: tree

...
code. The code works on the few test cases I have tried. Feedback appreciated.

{-# LANGUAGE ViewPatterns #-} module Parenthesis where import BasicPrelude hiding (concat,null,empty)

import Data.Sequence hiding (length) import Data.Foldable hiding (length,null)

balanceParens :: String -> String balanceParens str = go str [] empty where go [] [] (null -> True) = [] go [] [] parens = Data.Foldable.toList parens go ('(':xs) [] (null -> True) = go xs [RP] (singleton '(') go (')':xs) [] (null -> True) = go xs [] (fromList "()") go ('(':xs) debit parens = go xs (RP:debit) (parens |> '(') go (')':xs) [] parens = go xs [] corrected where corrected = ('(' <| parens) |> ')' go (')':xs) (RP:debit) parens = go xs debit (parens |> ')') go (_:xs) debit parens = go xs debit parens go [] (RP:debit) parens = go [] debit (parens |> ')')

example:

balanceParens "))(" "(())()" balanceParens ")))" "((()))"

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

Michael Litchard

2:04 p.m.

David, Am I mistaken in believing the code I have in both O(1) in time and space? I'm certain that my use of Data.Sequence, and therefore finger trees, has this code at O(1) time. Doesn't using a lazy list mean I am using O(1) space? Or, would I have to use the conduits or pipes library? On Tue, Mar 21, 2017 at 10:52 AM, David Turner < dct25-561bs@mythic-beasts.com> wrote:

...

... or it could be a ploy to see how you deal with incomplete or vague specs; how you ask clarifying questions etc.

Adding sufficiently many opening parens at the start can't obviously be done without traversing the whole input first, whereas adding them as you discover excesses of closing parens is possible to implement in a streaming fashion, i.e. with O(1) memory usage and only traversing the input once.

On 21 Mar 2017 17:44, "Michael Litchard" wrote:

I got this idea from looking at glassdoor comments of previous interviewees. The spec was vague, but I imagined that the requirement would need to be efficient and keep contiguous parenthesis unaltered. so balanceParens ")))" == "()()()" would be incorrect.

On Tue, Mar 21, 2017 at 10:20 AM, David Turner < dct25-561bs@mythic-beasts.com> wrote:

...
My attempt to replicate the OP's strategy but not impl:

balanceParens :: String -> String balanceParens s = replicate neededOpening '(' ++ s ++ replicate neededClosing ')' where depthChange '(' = 1 depthChange ')' = -1 depthChange _ = 0

depths = scanl (+) 0 $ map depthChange s neededOpening = negate $ minimum depths neededClosing = last depths + neededOpening

On 21 March 2017 at 17:16, David McBride wrote:

...
Whether your algorithm is correct depends on how you are supposed to rebalance them. My naive attempt gives very different results.

bal :: String -> String bal = go 0 where go :: Int -> String -> String go 0 "" = "" go n "" = replicate n ')' go n ('(':xs) = '(' : (go (n + 1) xs) go 0 (')':xs) = '(' : ')' : (go 0 xs) go n (')':xs) = ')' : (go (n - 1) xs)

bal "))(" "()()()"

bal ")))" "()()()"

...
I'm prepping for a coding interview, and am examining the task of correcting unbalanced parentheses. The finger tree seems to be the right data structure. As a proof of concept I've used Data.Sequence to test my idea. If this is the right direction to go, I'll write more specialized finger

On Tue, Mar 21, 2017 at 12:53 PM, Michael Litchard wrote: tree

...
code. The code works on the few test cases I have tried. Feedback appreciated.

{-# LANGUAGE ViewPatterns #-} module Parenthesis where import BasicPrelude hiding (concat,null,empty)

import Data.Sequence hiding (length) import Data.Foldable hiding (length,null)

balanceParens :: String -> String balanceParens str = go str [] empty where go [] [] (null -> True) = [] go [] [] parens = Data.Foldable.toList parens go ('(':xs) [] (null -> True) = go xs [RP] (singleton '(') go (')':xs) [] (null -> True) = go xs [] (fromList "()") go ('(':xs) debit parens = go xs (RP:debit) (parens |> '(') go (')':xs) [] parens = go xs [] corrected where corrected = ('(' <| parens) |> ')' go (')':xs) (RP:debit) parens = go xs debit (parens |> ')') go (_:xs) debit parens = go xs debit parens go [] (RP:debit) parens = go [] debit (parens |> ')')

example:

balanceParens "))(" "(())()" balanceParens ")))" "((()))"

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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David Turner

2:12 p.m.

It can't be done in better than O(n) time as you have to look at the whole input. I think you must be using O(n) space too. Consider a string of the form "()()()()...()())": You know the output starts with a '(' but you cannot know the second character of the output until you have read all the way to the end of the string and found the final character is unmatched. On 21 Mar 2017 18:04, "Michael Litchard" wrote:

...

David,

Am I mistaken in believing the code I have in both O(1) in time and space? I'm certain that my use of Data.Sequence, and therefore finger trees, has this code at O(1) time. Doesn't using a lazy list mean I am using O(1) space? Or, would I have to use the conduits or pipes library?

On Tue, Mar 21, 2017 at 10:52 AM, David Turner < dct25-561bs@mythic-beasts.com> wrote:

...
... or it could be a ploy to see how you deal with incomplete or vague specs; how you ask clarifying questions etc.

Adding sufficiently many opening parens at the start can't obviously be done without traversing the whole input first, whereas adding them as you discover excesses of closing parens is possible to implement in a streaming fashion, i.e. with O(1) memory usage and only traversing the input once.

On 21 Mar 2017 17:44, "Michael Litchard" wrote:

I got this idea from looking at glassdoor comments of previous interviewees. The spec was vague, but I imagined that the requirement would need to be efficient and keep contiguous parenthesis unaltered. so balanceParens ")))" == "()()()" would be incorrect.

On Tue, Mar 21, 2017 at 10:20 AM, David Turner < dct25-561bs@mythic-beasts.com> wrote:

...
My attempt to replicate the OP's strategy but not impl:

balanceParens :: String -> String balanceParens s = replicate neededOpening '(' ++ s ++ replicate neededClosing ')' where depthChange '(' = 1 depthChange ')' = -1 depthChange _ = 0

depths = scanl (+) 0 $ map depthChange s neededOpening = negate $ minimum depths neededClosing = last depths + neededOpening

On 21 March 2017 at 17:16, David McBride wrote:

...
Whether your algorithm is correct depends on how you are supposed to rebalance them. My naive attempt gives very different results.

bal :: String -> String bal = go 0 where go :: Int -> String -> String go 0 "" = "" go n "" = replicate n ')' go n ('(':xs) = '(' : (go (n + 1) xs) go 0 (')':xs) = '(' : ')' : (go 0 xs) go n (')':xs) = ')' : (go (n - 1) xs)

bal "))(" "()()()"

bal ")))" "()()()"

...
I'm prepping for a coding interview, and am examining the task of correcting unbalanced parentheses. The finger tree seems to be the right data structure. As a proof of concept I've used Data.Sequence to test my idea. If this is the right direction to go, I'll write more specialized finger

On Tue, Mar 21, 2017 at 12:53 PM, Michael Litchard wrote: tree

...
code. The code works on the few test cases I have tried. Feedback appreciated.

{-# LANGUAGE ViewPatterns #-} module Parenthesis where import BasicPrelude hiding (concat,null,empty)

import Data.Sequence hiding (length) import Data.Foldable hiding (length,null)

balanceParens :: String -> String balanceParens str = go str [] empty where go [] [] (null -> True) = [] go [] [] parens = Data.Foldable.toList parens go ('(':xs) [] (null -> True) = go xs [RP] (singleton '(') go (')':xs) [] (null -> True) = go xs [] (fromList "()") go ('(':xs) debit parens = go xs (RP:debit) (parens |> '(') go (')':xs) [] parens = go xs [] corrected where corrected = ('(' <| parens) |> ')' go (')':xs) (RP:debit) parens = go xs debit (parens |> ')') go (_:xs) debit parens = go xs debit parens go [] (RP:debit) parens = go [] debit (parens |> ')')

example:

balanceParens "))(" "(())()" balanceParens ")))" "((()))"

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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Richard A. O'Keefe

22 Mar 22 Mar

9:22 p.m.

I note that Ying's thesis on repairing bracket structure in XML is a cubic time algorithm. It's not immediately obvious to me that an algorithm computing "optimal" repairs for bracket structures can be faster than quadratic time. Everything hinges on what the requirements actually are. space?

...

I'm certain that my use of Data.Sequence, and therefore finger trees, has this code at O(1) time. Doesn't using a lazy list mean I am using O(1) space?

If the input has m characters and the output has n characters, the algorithm MUST take at least O(m+n) time. Two people have independently proposed basically the same streaming algorithm taking O(m+n) time and O(1) space with lazy Strings, and one of them proposed an algorithm that would reproduce the few test cases you provided, still with O(m+n) time and O(1) space, without finger or any other trees. Negotiating the specification comes first.

Richard A. O'Keefe

9:15 p.m.

...

On 22/03/2017, at 6:44 AM, Michael Litchard wrote:

I got this idea from looking at glassdoor comments of previous interviewees. The spec was vague, but I imagined that the requirement would need to be efficient and keep contiguous parenthesis unaltered. so balanceParens ")))" == "()()()" would be incorrect.

One parenthesIs two parenthesEs. Perhaps you could link to those glassdoor comments? It really is not obvious to me that "keep[ing] contiguous parenthes[e]s unaltered" even makes sense, let alone being desirable. Entirely by coincidence, I had a parenthesis problem about an hour ago, where I had changed fabs(X-(k-1)) to fabs(d)) and the right repair was to delete one of the right parentheses. The point of the question may well be not the code you come up with but the questions you ask on your way. I'd probably ask questions like these: 1. Do you have a precise specification? 2. Are you looking for an obviously correct solution, a solution with small workspace, a fast solution, or what? 3. What operations am I allowed to perform on a string? Which characters may I insert? Which characters may I delete? Is transposition of adjacent characters allowed? 4. Are you looking for a repair of minimal cost in some sense? If so, how is that cost computed? For example, inserting ( might cost 2 units and deleting ) might cost 3 units. 5. There's been a lot of work done on least cost syntax repair (e.g., Roland Backhouse). Am I allowed to look that up?

Michael Orlitzky

21 Mar 21 Mar

1:23 p.m.

New subject: Code Review Request - Unbalanced Parenthesis correction

On 03/21/2017 01:16 PM, David McBride wrote:

...

Whether your algorithm is correct depends on how you are supposed to rebalance them. My naive attempt gives very different results.

You guys are trying to hard. Here's an algorithm that meets the stated requirements: balance _ = ""

Andreas Abel

24 Mar 24 Mar

6:30 a.m.

New subject: Code Review Request - Unbalanced Parenthesis correction

...

balance _ = "" Haha!

The other trivial solution simply replaces each parenthesis by "()": balance = concatMap $ const "()" balance "))(" then returns "()()()". This achieves that the balanced string is a superstring of the original string. (Not a minimal one.) On 21.03.2017 18:23, Michael Orlitzky wrote:

...

On 03/21/2017 01:16 PM, David McBride wrote:

...
Whether your algorithm is correct depends on how you are supposed to rebalance them. My naive attempt gives very different results.

You guys are trying to hard. Here's an algorithm that meets the stated requirements:

balance _ = "" -- Andreas Abel <>< Du bist der geliebte Mensch.

Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel@gu.se http://www.cse.chalmers.se/~abela/

MarLinn

21 Mar 21 Mar

1:49 p.m.

New subject: Code Review Request - Unbalanced Parenthesis correction

Hi Michael, I think you're making your own task harder than necessary. For one thing, -XViewPatterns is nice, but in this case they hide some of the structure. Most importantly, they hide that all the special null-cases are actually unnecessary because the normal cases already cover them. I would further advise to use layout to reveal even more structure. That's especially useful when you later convert the explicit recursion into a fold. But even then, on a different level you're still working too hard: You're parsing the string and building/correcting the tree in one step. Why not create the tree, convert the tree, and then read out the tree in three steps? It's still the same complexity class but much easier to write and read. And once you are free to think of the tree manipulations on their own it might help recognize optimizations like those the solutions of other commenters use. That's not to say your modifications are useless. But the exploratory phase seems too early to apply them. Cheers, MarLinn On 2017-03-21 17:53, Michael Litchard wrote:

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I'm prepping for a coding interview, and am examining the task of correcting unbalanced parentheses. The finger tree seems to be the right data structure. As a proof of concept I've used |Data.Sequence| to test my idea. If this is the right direction to go, I'll write more specialized finger tree code. The code works on the few test cases I have tried. Feedback appreciated.

|{-# LANGUAGE ViewPatterns #-}moduleParenthesis whereimportBasicPrelude hiding (concat,null,empty)importData.Sequence hiding (length)importData.Foldable hiding (length,null)balanceParens ::String ->String balanceParens str =go str []empty wherego [][](null ->True)=[]go [][]parens =Data.Foldable.toList parens go ('(':xs)[](null ->True)=go xs [RP](singleton '(')go (')':xs)[](null ->True)=go xs [](fromList "()")go ('(':xs)debit parens =go xs (RP:debit)(parens |>'(')go (')':xs)[]parens =go xs []corrected wherecorrected =('('<|parens)|>')'go (')':xs)(RP:debit)parens =go xs debit (parens |>')')go (_:xs)debit parens =go xs debit parens go [](RP:debit)parens =go []debit (parens |>')')|

example:

|balanceParens "))(""(())()"balanceParens ")))""((()))"|

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Richard A. O'Keefe

22 Mar 22 Mar

8:52 p.m.

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Andreas Abel
David McBride
David Turner
MarLinn
Michael Litchard
Michael Orlitzky
Richard A. O'Keefe

Code Review Request - Unbalanced Parenthesis correction

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