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Merge pull request #1110 from pwadsworth/ADTs
Concept Algebraic Data Types with exercise
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‎concepts/algebraic-data-types/.meta/config.json

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{
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"authors": [
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"pwadsworth"
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],
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"blurb": "Introduction to Algebraic Data Types in Haskell."
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}

‎concepts/algebraic-data-types/about.md

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# Introduction
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An Algebraic Data Type (ADT) represents a fixed number of named cases.
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Each value of an ADT corresponds to exactly one of the named cases.
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An ADT is defined using the `data` keyword, with cases separated by pipe (`|`) characters.
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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_).
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```haskell
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data Season
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= Spring
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| Summer
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| Autumn
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| Winter
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```
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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.
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```haskell
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data Number
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= NInt Int --'NInt' is the constructor for an Int Number.
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| NFloat Float --'NFloat' is the constructor for an Float Number.
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| Invalid --'Invalid' does not have data associated to it.
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```
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Creating a value for a specific case can be done by referring to its name (e.g, `NInt 22`).
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As case names are just constructor functions, associated data can be passed as a regular function argument.
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ADTs have _structural equality_, which means that two values for the same case and with the same (optional) data are equivalent.
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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:
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```haskell
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add1 :: Number -> String
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add1 number =
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case number of
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NInt i -> show (i + 1)
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NFloat f -> show (f + 1.0)
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Invalid -> error "Invalid input"
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```

‎concepts/algebraic-data-types/introduction.md

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# Introduction
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An Algebraic Data Type (ADT) represents a fixed number of named cases.
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Each value of an ADT corresponds to exactly one of the named cases.
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An ADT is defined using the `data` keyword, with cases separated by pipe (`|`) characters.
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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_).
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```haskell
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data Season
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= Spring
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| Summer
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| Autumn
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| Winter
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```
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Each case of an ADT can optionally have data associated with it, and different cases can have different types of data.
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When the case has data associated, a constructor is required.
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```haskell
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data Number
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= NInt Int --'NInt' is the constructor for an Int Number.
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| NFloat Float --'NFloat' is the constructor for an Float Number.
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| Invalid --'Invalid' does not have data associated to it.
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```
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Creating a value for a specific case can be done by referring to its name (e.g, `NInt 22`).
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As case names are just constructor functions, associated data can be passed as a regular function argument.
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ADTs have _structural equality_, which means that two values for the same case and with the same (optional) data are equivalent.
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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:
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```haskell
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add1 :: Number -> String
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add1 number =
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case number of
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NInt i -> show (i + 1)
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NFloat f -> show (f + 1.0)
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Invalid -> error "Invalid input"
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```

‎concepts/algebraic-data-types/introduction.md.tpl

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# Introduction
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%{concept:algebraic-data-types}

‎concepts/algebraic-data-types/links.json

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[
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{
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"url": "https://learnyouahaskell.github.io/making-our-own-types-and-typeclasses.html#algebraic-data-types",
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"description": "Algebraic Data Types"
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},
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{
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"url": "https://learnyouahaskell.github.io/making-our-own-types-and-typeclasses.html#type-parameters",
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"description": "Type variables"
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}
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]

‎config.json

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"concepts": [
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"pattern-matching-literals"
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]
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},
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{
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"slug": "valentines-day",
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"name": "Valentines Day",
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"uuid": "537d8df3-0b12-4dbe-aa86-65ae79c4de0e",
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"prerequisites": [
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"pattern-matching-literals",
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"numbers"
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],
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"status": "beta",
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"concepts": [
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"algebraic-data-types"
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]
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}
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],
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"practice": [
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"uuid": "ca21a553-6fc1-49be-8f47-730f97330862",
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"slug": "pattern-matching-literals",
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"name": "Pattern Matching Literals"
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},
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{
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"uuid": "cb756cfd-9b5c-408d-bf3f-b2adc8322da3",
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"slug": "algebraic-data-types",
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"name": "Algebraic Data Types"
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}
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],
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"key_features": [

‎exercises/concept/valentines-day/.docs/hints.md

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# Hints
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## 1. Define the approval
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- [Define the algebraic data type][ADT] `Approval` with constructors for the required options.
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## 2. Define the cuisine
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- [Define the algebraic data type][ADT] `Cuisine` with constructors for the required options.
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## 3. Define the movie genres
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- [Define the algebraic data type][ADT] `Genre` with constructors for the required options.
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## 4. Define the activity
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- [Define an algebraic data type with associated data][ADT-with-data] to encapsulate the different activities.
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## 5. Rate the activity
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- The best way to execute logic based on the activity's value is to use [case expressions][case-expression].
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- Pattern matching an algebraic data type case provides access to its associated data.
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- To add an additional condition to a pattern, you can use a [guard][guards] inside a case.
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- If you want to catch all other possible values in one case, you can use the wildcard pattern `_`.
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[ADT]: https://www.schoolofhaskell.com/school/starting-with-haskell/introduction-to-haskell/2-algebraic-data-types#enumeration-types
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[ADT-with-data]: https://www.schoolofhaskell.com/school/starting-with-haskell/introduction-to-haskell/2-algebraic-data-types#beyond-enumerations
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[case-expression]: https://www.schoolofhaskell.com/school/starting-with-haskell/introduction-to-haskell/2-algebraic-data-types#case-expessions
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[guards]: https://learnyouahaskell.github.io/syntax-in-functions.html#guards-guards

‎exercises/concept/valentines-day/.docs/instructions.md

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# Instructions
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In this exercise, it's Valentine's day and you are planning what to do with your partner.
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Your partner has lots of ideas, and is asking you to rate the ideas, in order to find the best activity.
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The following ideas are proposed by your partner:
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- Playing a board game
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- Chill out
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- Watch a movie
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- Go to a restaurant
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- Take a walk
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You have six tasks to help choose your Valentine's day activity.
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## 1. Define the approval
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For each idea your partner proposes, you respond with one of three options: yes, no or maybe.
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Define the `Approval` algebraic data type to represent these options for the following three cases: `Yes`, `No` or `Maybe`.
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## 2. Define the cuisines
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Your partner has selected two possible restaurants: one based on Korean cuisine and the other based on Turkish cuisine.
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Define the `Cuisine` algebraic data type to represent these restaurants as the following two cases: `Korean` or `Turkish`.
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## 3. Define the movie genres
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There are tons of movies to choose from, so to narrow things down, your partner also lists their preferred genre.
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Define the `Genre` algebraic data type to represent the following genres cases: `Crime`, `Horror`, `Romance` or `Thriller`.
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## 4. Define the activity
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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.
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Define the `Activity` algebraic data type to represent these activity types:
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- `BoardGame`: no associated data.
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- `Chill`: no associated data.
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- `Movie`: has its `Genre` as associated data.
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- `Restaurant`: has its `Cuisine` as associated data.
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- `Walk`: has an `Int` representing the number of kilometers to walk as associated data.
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## 5. Rate the activity
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Finally, you're ready to rate your partner's ideas.
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This is how you feel about your partner's idea:
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- Playing a board game: no.
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- Chill out: no.
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- Watch a movie: yes if it is a romantic movie; otherwise, no.
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- Go to a restaurant: yes if the cuisine is Korean, maybe if it is Turkish.
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- Take a walk: yes if the walk is less than three kilometers; maybe if it is between three and five kilometers (inclusive); otherwise, no.
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Implement a function named `rateActivity` that takes an `Activity` value and returns the `Approval` based on the above sentiments.
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For example:
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```haskell
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rateActivity (Restaurant Turkish)
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-- -> Maybe
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```

‎exercises/concept/valentines-day/.docs/introduction.md

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# Introduction
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An Algebraic Data Type (ADT) represents a fixed number of named cases.
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Each value of an ADT corresponds to exactly one of the named cases.
5+
6+
An ADT is defined using the `data` keyword, with cases separated by pipe (`|`) characters.
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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_).
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```haskell
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data Season
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= Spring
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| Summer
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| Autumn
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| Winter
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```
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Each case of an ADT can optionally have data associated with it, and different cases can have different types of data.
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When the case has data associated, a constructor is required.
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```haskell
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data Number
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= NInt Int --'NInt' is the constructor for an Int Number.
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| NFloat Float --'NFloat' is the constructor for an Float Number.
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| Invalid --'Invalid' does not have data associated to it.
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```
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Creating a value for a specific case can be done by referring to its name (e.g, `NInt 22`).
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As case names are just constructor functions, associated data can be passed as a regular function argument.
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ADTs have _structural equality_, which means that two values for the same case and with the same (optional) data are equivalent.
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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:
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```haskell
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add1 :: Number -> String
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add1 number =
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case number of
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NInt i -> show (i + 1)
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NFloat f -> show (f + 1.0)
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Invalid -> error "Invalid input"
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```

‎exercises/concept/valentines-day/.meta/DONT-TEST-STUB

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Teaches the basics of defining data types, and we found that people usually learn best when they have to write things from scratch.
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Thus, the stub lacks the data type definitions for the tests.

‎exercises/concept/valentines-day/.meta/config.json

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{
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"blurb": "Learn about algebraic data types by deciding what activity to surprise your partner with on Valentines Day.",
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"authors": [
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"pwadsworth"
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],
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"forked_from": [
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"fsharp/valentines-day"
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],
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"files": {
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"solution": [
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"src/ValentinesDay.hs",
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"package.yaml"
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],
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"test": [
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"test/Tests.hs"
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],
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"exemplar": [
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".meta/exemplar/src/ValentinesDay.hs"
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],
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"invalidator": [
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"stack.yaml"
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]
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}
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}

‎exercises/concept/valentines-day/.meta/design.md

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# Design
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## Learning objectives
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- Know what [Algebraic Data Types][ADT] (ADT) are.
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- Know how ADTs are different from enums.
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- Know how to define ADT, with and without data.
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- Know how to pattern match on ADTs using [case expressions][case-expression].
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## Out of scope
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- Recursive ADT.
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- Single type wrapper ADT.
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- Active patterns.
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- Adding members to ADT.
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- `function` pattern shorthand notation.
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## Concepts
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- `Algebraic Data Types`: know what ADTs are; know how ADTs are different from enums; know how to define an ADT, with and without data; know how to pattern match on ADTs.
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## Prerequisites
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- `basics`: defining functions and scoping and using integers.
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- `pattern-matching`: know how to do pattern matching.
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[ADT]: https://www.schoolofhaskell.com/school/starting-with-haskell/introduction-to-haskell/2-algebraic-data-types#enumeration-types
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[case-expression]: https://www.schoolofhaskell.com/school/starting-with-haskell/introduction-to-haskell/2-algebraic-data-types#case-expessions

‎exercises/concept/valentines-day/.meta/exemplar/package.yaml

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name: valentines-day
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version: 1.0.0.0
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dependencies:
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- base
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library:
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exposed-modules: ValentinesDay
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source-dirs: src
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ghc-options: -Wall
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tests:
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test:
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main: Tests.hs
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source-dirs: test
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dependencies:
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- valentines-day
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- hspec

‎exercises/concept/valentines-day/.meta/exemplar/src/ValentinesDay.hs

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module ValentinesDay (rateActivity, Approval (..), Cuisine (..), Genre (..), Activity (..)) where
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data Approval
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= Yes
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| No
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| Maybe
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data Cuisine
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= Korean
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| Turkish
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data Genre
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= Crime
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| Horror
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| Romance
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| Thriller
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data Activity
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= BoardGame
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| Chill
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| Movie Genre
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| Restaurant Cuisine
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| Walk Int
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rateActivity :: Activity -> Approval
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rateActivity activity =
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case activity of
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Restaurant Korean -> Yes
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Restaurant Turkish -> Maybe
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Movie Romance -> Yes
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Movie _ -> No
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Walk kilometers
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| kilometers < 3 -> Yes
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| kilometers <= 5 -> Maybe
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_ -> No

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