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to Matching Brackets in the Elm Track

Published at Apr 20 2020 · 0 comments
Instructions
Test suite
Solution

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

Elm Installation

Refer to the Installing Elm page for information about installing elm.

Writing the Code

The code you have to write is located inside the src/ directory of the exercise. Elm automatically installs packages dependencies the first time you run the tests so we can start by running the tests from the exercise directory with:

$ elm-test

To automatically run tests again when you save changes:

$ elm-test --watch

As you work your way through the tests suite in the file tests/Tests.elm, be sure to remove the skip <| calls from each test until you get them all passing!

Source

Ginna Baker

Submitting Incomplete Solutions

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

Tests.elm

module Tests exposing (tests)

import Expect
import MatchingBrackets exposing (isPaired)
import String
import Test exposing (..)


tests : Test
tests =
    describe "MatchingBrackets"
        [ test "paired square brackets" <|
            \_ ->
                Expect.equal True (isPaired "[]")
        , skip <|
            test "empty string" <|
                \_ ->
                    Expect.equal True (isPaired "")
        , skip <|
            test "unpaired brackets" <|
                \_ ->
                    Expect.equal False (isPaired "[[")
        , skip <|
            test "wrong ordered brackets" <|
                \_ ->
                    Expect.equal False (isPaired "}{")
        , skip <|
            test "wrong closing bracket" <|
                \_ ->
                    Expect.equal False (isPaired "{]")
        , skip <|
            test "paired with whitespace" <|
                \_ ->
                    Expect.equal True (isPaired "{ }")
        , skip <|
            test "partially paired brackets" <|
                \_ ->
                    Expect.equal False (isPaired "{[])")
        , skip <|
            test "simple nested brackets" <|
                \_ ->
                    Expect.equal True (isPaired "{[]}")
        , skip <|
            test "seceral paired brackets" <|
                \_ ->
                    Expect.equal True (isPaired "{}[]")
        , skip <|
            test "paired and nested brackets" <|
                \_ ->
                    Expect.equal True (isPaired "([{}({}[])])")
        , skip <|
            test "unopened closing brackets" <|
                \_ ->
                    Expect.equal False (isPaired "{[)][]}")
        , skip <|
            test "unpaired and nested brackets" <|
                \_ ->
                    Expect.equal False (isPaired "([{])")
        , skip <|
            test "paired and wrong nested brackets" <|
                \_ ->
                    Expect.equal False (isPaired "[({]})")
        , skip <|
            test "math expression" <|
                \_ ->
                    Expect.equal True (isPaired "(((185 + 223.85) * 15) - 543)/2")
        , skip <|
            test "complex latex expression" <|
                \_ ->
                    Expect.equal True (isPaired "\\left(\\begin{array}{cc} \\frac{1}{3} & x\\\\ \\mathrm{e}^{x} &... x^2 \\end{array}\\right)")
        ]
module MatchingBrackets exposing (isPaired)

import Stack exposing (Stack)


brackets : List ( Char, Char )
brackets =
    [ ( '[', ']' ), ( '(', ')' ), ( '{', '}' ) ]


isFirstEq : Char -> ( Char, Char ) -> Bool
isFirstEq char pair =
    Tuple.first pair == char


isSecondEq : Char -> ( Char, Char ) -> Bool
isSecondEq char pair =
    Tuple.second pair == char


isOpeningBracket : Char -> Bool
isOpeningBracket char =
    List.any (isFirstEq char) brackets


isClosingBracket : Char -> Bool
isClosingBracket char =
    List.any (isSecondEq char) brackets


getPairForClosingBracket : Char -> Maybe Char
getPairForClosingBracket char =
    brackets
        |> find (isSecondEq char)
        |> Maybe.map Tuple.first


{-| List.find, borrowed from List.extra
-}
find : (a -> Bool) -> List a -> Maybe a
find predicate list =
    case list of
        [] ->
            Nothing

        first :: rest ->
            if predicate first then
                Just first

            else
                find predicate rest


checkChar : Char -> Stack Char -> Stack Char
checkChar char stack =
    if isOpeningBracket char then
        Stack.push char stack

    else if isClosingBracket char then
        if Stack.top stack == getPairForClosingBracket char then
            Tuple.second (Stack.pop stack)

        else
            -- a small hack, closing bracket would never be removed
            Stack.push char stack

    else
        stack


isPaired : String -> Bool
isPaired input =
    input
        |> String.foldl checkChar Stack.initialise
        |> Stack.toList
        |> List.isEmpty

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