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Carter0's solution

to Matching Brackets in the Elm Track

Published at Dec 23 2019 · 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)


isPaired : String -> Bool
isPaired input =
    case String.foldl stackFunctionality (Result.Ok []) input of
        Err error ->
            False

        Ok value ->
            List.isEmpty value

 {- 
 TLDR Andthen : if any of the updateStack function calls returns a Err, Err the whole thing.
 Long version: Use currying to make a callback function that matches the function signature for andThen.
 Run the code with andthen, which first checks to see if Result is already an error, if it is, don't even
 bother running the callback function, just return the error.
 Otherwise run the callback function and return whatever that result happens to be.
 -}
stackFunctionality : Char -> Result String (List Char) -> Result String (List Char) 
stackFunctionality bracket bracketStack =
    Result.andThen (updateStack bracket) bracketStack

updateStack : Char -> List Char -> Result String (List Char)
updateStack bracket bracketStack =
    if isStartingBrackets bracket then
        Result.Ok <| bracket :: bracketStack

    else if isEndingBrackets bracket then
        let
            popValue =
                List.head bracketStack
        in
        case popValue of
            Nothing ->
                Result.Err "Nothing to pop off in stack"

            Just a ->
                if bracketToLookFor a == bracket then
                    Result.Ok <| List.drop 1 bracketStack
                    -- Matching bracket, return the rest

                else
                    Result.Err "Brackets do not match"

    else
        Result.Ok bracketStack


isEndingBrackets : Char -> Bool
isEndingBrackets char =
    char == '}' || char == ']' || char == ')'


isStartingBrackets : Char -> Bool
isStartingBrackets char =
    char == '{' || char == '[' || char == '('


bracketToLookFor : Char -> Char
bracketToLookFor char =
    if char == '{' then
        '}'

    else if char == '(' then
        ')'

    else
        ']'

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