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Valentines Day
Valentines Day

Valentines Day

Learning Exercise
OverviewCommunity Solutions

Introduction

An Algebraic Data Type (ADT) represents a fixed number of named cases. Each value of an ADT corresponds to exactly one of the named cases.

An ADT is defined using the data keyword, with cases separated by pipe (|) characters. If none of the cases have data associated with them the ADT is similar to what other languages usually refer to as an enumeration (or enum).

data Season
  = Spring
  | Summer
  | Autumn
  | Winter

Each case of an ADT can optionally have data associated with it, and different cases can have different types of data. When the case has data associated, a constructor is required.

data Number
  = NInt Int      --'NInt' is the constructor for an Int Number.
  | NFloat Float  --'NFloat' is the constructor for an Float Number.
  | Invalid       --'Invalid' does not have data associated to it.

Creating a value for a specific case can be done by referring to its name (e.g, NInt 22). As case names are just constructor functions, associated data can be passed as a regular function argument.

ADTs have structural equality, which means that two values for the same case and with the same (optional) data are equivalent.

While one can use if/else expressions to work with ADTs, the recommended way to work with them is through pattern matching using case statement:

add1 :: Number -> String
add1 number =
    case number of
      NInt    i -> show (i + 1)
      NFloat  f -> show (f + 1.0)
      Invalid   -> error "Invalid input"

Instructions

In this exercise, it's Valentine's day and you are planning what to do with your partner. Your partner has lots of ideas, and is asking you to rate the ideas, in order to find the best activity.

The following ideas are proposed by your partner:

  • Playing a board game
  • Chill out
  • Watch a movie
  • Go to a restaurant
  • Take a walk

You have six tasks to help choose your Valentine's day activity.

1. Define the approval

For each idea your partner proposes, you respond with one of three options: yes, no or maybe.

Define the Approval algebraic data type to represent these options for the following three cases: Yes, No or Maybe.

2. Define the cuisines

Your partner has selected two possible restaurants: one based on Korean cuisine and the other based on Turkish cuisine.

Define the Cuisine algebraic data type to represent these restaurants as the following two cases: Korean or Turkish.

3. Define the movie genres

There are tons of movies to choose from, so to narrow things down, your partner also lists their preferred genre.

Define the Genre algebraic data type to represent the following genres cases: Crime, Horror, Romance or Thriller.

4. Define the activity

As mentioned, your partner has come up with five possible activities: playing a board game, chill out, watch a movie, go to a restaurant and taking a walk.

Define the Activity algebraic data type to represent these activity types:

  • BoardGame: no associated data.
  • Chill: no associated data.
  • Movie: has its Genre as associated data.
  • Restaurant: has its Cuisine as associated data.
  • Walk: has an Int representing the number of kilometers to walk as associated data.

5. Rate the activity

Finally, you're ready to rate your partner's ideas. This is how you feel about your partner's idea:

  • Playing a board game: no.
  • Chill out: no.
  • Watch a movie: yes if it is a romantic movie; otherwise, no.
  • Go to a restaurant: yes if the cuisine is Korean, maybe if it is Turkish.
  • Take a walk: yes if the walk is less than three kilometers; maybe if it is between three and five kilometers (inclusive); otherwise, no.

Implement a function named rateActivity that takes an Activity value and returns the Approval based on the above sentiments. For example:

rateActivity (Restaurant Turkish)
-- -> Maybe
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