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# stefanscheidt's solution

## to Bracket Push in the PureScript Track

Published at Mar 25 2019 · 0 comments
Instructions
Test suite
Solution

Given a string containing brackets [], braces {} and parentheses (), verify that all the pairs are matched and nested correctly.

Ginna Baker

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### Main.purs

module Test.Main where

import Prelude

import Effect (Effect)
import Test.Unit.Assert as Assert
import Test.Unit (TestSuite, suite, test)
import Test.Unit.Main (runTest)
import BracketPush (isPaired)

main :: Effect Unit
main = runTest suites

suites :: TestSuite
suites = do
suite "BracketPush.isPaired" do

test "paired square brackets" $Assert.equal true (isPaired "[]") test "empty string"$
Assert.equal true
(isPaired "")

test "unpaired brackets" $Assert.equal false (isPaired "[[") test "wrong ordered brackets"$
Assert.equal false
(isPaired "}{")

test "paired with whitespace" $Assert.equal true (isPaired "{ }") test "simple nested brackets"$
Assert.equal true
(isPaired "{[]}")

test "several paired brackets" $Assert.equal true (isPaired "{}[]") test "paired and nested brackets"$
Assert.equal true
(isPaired "([{}({}[])])")

test "unopened closing brackets" $Assert.equal false (isPaired "{[)][]}") test "unpaired and nested brackets"$
Assert.equal false
(isPaired "([{])")

test "paired and wrong nested brackets" $Assert.equal false (isPaired "[({]})") test "math expression"$
Assert.equal true
(isPaired "(((185 + 223.85) * 15) - 543)/2")

test "complex latex expression" \$
Assert.equal true
(isPaired "\\left(\\begin{array}{cc} \\frac{1}{3} & x\\\\ \\mathrm{e}^{x} &... x^2 \\end{array}\\right)")
module BracketPush
( isPaired
) where

import Prelude

import Data.Foldable (elem)
import Data.List (List(..), filter, fromFoldable, head, tail)
import Data.Maybe (Maybe(..))
import Data.String.CodeUnits (toCharArray)

isPaired :: String -> Boolean
isPaired = eq (Just Nil) <<< parse <<< normalize

normalize :: String -> List Char
normalize = filter isBracket <<< fromFoldable <<< toCharArray

parse :: List Char -> Maybe (List Char)
parse Nil = Just Nil
parse (Cons '(' xs) = parseBrackets ')' xs
parse (Cons '[' xs) = parseBrackets ']' xs
parse (Cons '{' xs) = parseBrackets '}' xs
parse inp@(Cons x xs)
| elem x [']','}', ')'] = Just inp
| otherwise             = Nothing

parseBrackets :: Char -> List Char -> Maybe (List Char)
parseBrackets close xs = do
subtree <- parse xs
then tail subtree >>= parse
else Nothing

isBracket :: Char -> Boolean
isBracket = flip elem brackets
where
brackets = toCharArray "(){}[]"