🎉 Exercism Research is now launched. Help Exercism, help science and have some fun at research.exercism.io 🎉
Avatar of hendrikniemann

hendrikniemann's solution

to Bracket Push in the PureScript Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

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

Source

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.List (List(..), (:))
import Data.Maybe (Maybe(..))
import Data.String (uncons)
import Data.String.Regex (replace)
import Data.String.Regex.Flags (global)
import Data.String.Regex.Unsafe (unsafeRegex)

isOpen :: Char -> Boolean
isOpen c = c == '(' || c == '[' || c == '{'

matches :: Char -> Char -> Boolean
matches '(' ')' = true
matches '[' ']' = true
matches '{' '}' = true
matches _   _   = false

onlyBrackets :: String -> String
onlyBrackets = replace (unsafeRegex "[^()[\\]{}]" global) ""

isPaired :: String -> Boolean
isPaired = onlyBrackets >>> isPaired' Nil
  where
    isPaired' :: List Char -> String -> Boolean
    isPaired' Nil   ""  = true
    isPaired' stack str = case uncons str of
        Nothing -> false
        Just { head, tail } ->
            if isOpen head
            -- open brackets get pushed to stack
            then isPaired' (head:stack) tail
            -- closed brackets need to match the stack head
            else case stack of
                Nil -> false
                Cons x xs -> matches x head && isPaired' xs tail

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?