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

dbalmain's solution

to Bracket Push in the PureScript Track

Published at Jul 13 2018 · 0 comments
Test suite

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.


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 Data.Array (elem, foldM)
import Data.List (List(..), (:))
import Data.Maybe (Maybe(..))
import Data.String (toCharArray)
import Data.Tuple (Tuple(..), fst, lookup, snd)
import Prelude (flip, map, otherwise, ($), (<$>), (==))

isPaired ∷ String → Boolean
isPaired = isPaired' [ (Tuple '(' ')')
                     , (Tuple '[' ']')
                     , (Tuple '{' '}') ]

isPaired' :: Array (Tuple Char Char) → String → Boolean
isPaired' ps xs = Just Nil == (foldM f Nil $ toCharArray xs)
    f stack ch
      | ch `elem` map snd ps =
        case stack of
          (top : stack') -> if ch == top then Just stack' else Nothing
          _ -> Nothing
      | ch `elem` map fst ps = (flip (:) stack) <$> lookup ch ps
      | otherwise = Just stack

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?