Add record types concept and associated exercise

concepts/lists/about.md

@@ -5,15 +5,23 @@ As lists are immutable, once a list has been constructed, its value can never ch
 Elm lists have a _head_ (the first element) and a _tail_ (everything after the first element).
 The tail of a list is itself a list.
 
+Type annotations for lists can be defined as follows
+
+```elm
+List String --> a list of String
+List Int --> a list of Int
+```
+
 Lists can be defined as follows:
 
 ```elm
+empty : List Char
 empty = []
 
-singleValue = [ 5 ]
-singleValueAlternative = List.singleton 5
+singleValue = [ 5 ] --> List Int
+singleValueAlternative = List.singleton 5 --> List Int
 
-threeValues = [ "a", "b", "c" ]
+threeValues = [ "a", "b", "c" ] --> List String
 ```
 
 The most common way to add an element to a list is through the `::` (cons) operator:

concepts/records/.meta/config.json

@@ -0,0 +1,9 @@
+{
+  "blurb": "Learn how to use records in an Elm program",
+  "authors": [
+    "ceddlyburge"
+  ],
+  "contributors": [
+    "mpizenberg"
+  ]
+}

concepts/records/about.md

@@ -0,0 +1,160 @@
+# About
+
+[Records][records] are data structures grouping together related information with labels.
+They are similar to objects in JavaScript or Java, or structs in C or Rust, but with some key distinctions.
+
+## Creating records
+
+Records are [created][create] with curly brackets and their elements are separated by commas.
+
+```elm
+firefly =
+    { name = "Firefly"
+    , creator = "Joss Whedon"
+    , episodes = 14
+    }
+```
+
+The type of a record is also defined with a similar syntax, except equal signs `=` are replaced by colon signs `:`.
+
+```elm
+firefly : { name : String, creator : String, episodes : Int }
+firefly =
+    { name = "Firefly"
+    , creator = "Joss Whedon"
+    , episodes = 14
+    }
+```
+
+Repeating these type definitions is cumbersome, so using a [type alias][record-types] to share the definition is a common idiom.
+
+```elm
+type alias TvSeries =
+    { name : String
+    , creator : String
+    , episodes : Int
+    }
+
+firefly : TvSeries
+firefly =
+    { name = "Firefly"
+    , creator = "Joss Whedon"
+    , episodes = 14
+    }
+```
+
+The type alias also automatically supplies a constructor function for the record.
+This is sometimes surprising since functions in Elm must start with a lowercase character, but type constructors are one exception.
+
+```elm
+firefly : TvSeries
+firefly = TvSeries "Firefly" "Joss Whedon" 14
+```
+
+## Accessing record fields
+
+The main way to [access a field value of a record instance][access] is to use the dot-notation such as `firefly.creator`.
+The Elm compiler also supports special accessor functions starting with a dot `.` such as `.creator` that will work on any record with a field of that name.
+That is the second exception to the rule that functions must start with a lowercase character.
+In fact user-defined functions must start with a lowercase character, but the two examples we just discovered are generated by the compiler, not the programmer.
+
+```elm
+firefly : TvSeries
+firefly =
+    { name = "Firefly"
+    , creator = "Joss Whedon"
+    , episodes = 14
+    }
+
+firefly.name
+    --> "Firefly"
+
+.creator firefly
+    --> "Joss Whedon"
+```
+
+Records can also be _destructured_ in bindings using the [record pattern matching][pattern-matching].
+Destructuring works with any record that has fields of the relevant names.
+
+```elm
+episodesCount : TvSeries -> Int
+episodesCount { episodes } =
+    episodes
+
+complicatedCopy : TvSeries -> TvSeries
+complicatedCopy show =
+    let
+        { name, creator } = show
+    in
+    { name = name
+    , creator = creator
+    , episodes = episodesCount show
+    }
+```
+
+## Updating records
+
+Records are immutable, as everything in Elm, so once a record is defined, its field values can never change.
+There is a [special syntax to create a copy of an existing record, but changing one or more fields][update].
+This syntax uses the pipe symbol `|` to distinguish the original record and the fields that will change, such as `{ oldRecord | field1 = newValue }`.
+
+```elm
+firefly : TvSeries
+firefly =
+    { name = "Firefly"
+    , creator = "Joss Whedon"
+    , episodes = 14
+    }
+
+updatedFirefly : TvSeries
+updatedFirefly =
+    { firefly | creator = "J Whedon", episodes = 15 }
+```
+
+## Comparing records
+
+Elm uses [structural equality][equality], which means that two instances of the same record with identical values are equal.
+
+```elm
+firefly1 = TvSeries "Firefly" "Joss Whedon" 14
+firefly2 = TvSeries "Firefly" "Joss Whedon" 14
+
+firefly1 == firefly2
+    --> True
+```
+
+## Extensible records
+
+Elm also supports [structural typing][structural-typing] meaning that if a function requires a record with an x and y field, it will work with any record that has those fields such as 2D points, 3D points, spaceships, etc.
+Those record types have a pipe `|` in their definition, such as `{ a | x : Float, y : Float }` and are called "extensible records" in Elm terminology.
+Beware of the difference between a pipe `|` in a type definition, which is an extensible record definition (`{ a | x : Int }`), and a pipe `|` an actual record instance which means that we are updating some fields of that record.
+
+```elm
+point2d = { x = 1, y = 2 }
+point3d = { x = 3, y = 4, z = 7 }
+
+.x point2d --> 1
+.x point3d --> 3
+
+length : { a | x : Float, y : Float } -> Float
+length vector =
+    sqrt (vector.x * vector.x + vector.y * vector.y)
+
+length point2d --> 2.236068
+length point3d --> 5
+
+translateX : { a | x : Float } -> { a | x : Float }
+translateX vector =
+    { vector | x = vector.x + 1 }
+
+translateX point2d --> { x = 2, y = 2 }
+translateX point3d --> { x = 4, y = 4, z = 7 }
+```
+
+[records]: https://elm-lang.org/docs/records
+[record-types]: https://elm-lang.org/docs/records#record-types
+[structural-typing]: https://en.wikipedia.org/wiki/Structural_type_system
+[create]: https://elm-lang.org/docs/records#what-is-a-record-
+[update]: https://elm-lang.org/docs/records#updating-records
+[access]: https://elm-lang.org/docs/records#access
+[pattern-matching]: https://elm-lang.org/docs/records#pattern-matching

concepts/records/introduction.md

@@ -0,0 +1,160 @@
+# About
+
+[Records][records] are data structures grouping together related information with labels.
+They are similar to objects in JavaScript or Java, or structs in C or Rust, but with some key distinctions.
+
+## Creating records
+
+Records are [created][create] with curly brackets and their elements are separated by commas.
+
+```elm
+firefly =
+    { name = "Firefly"
+    , creator = "Joss Whedon"
+    , episodes = 14
+    }
+```
+
+The type of a record is also defined with a similar syntax, except equal signs `=` are replaced by colon signs `:`.
+
+```elm
+firefly : { name : String, creator : String, episodes : Int }
+firefly =
+    { name = "Firefly"
+    , creator = "Joss Whedon"
+    , episodes = 14
+    }
+```
+
+Repeating these type definitions is cumbersome, so using a [type alias][record-types] to share the definition is a common idiom.
+
+```elm
+type alias TvSeries =
+    { name : String
+    , creator : String
+    , episodes : Int
+    }
+
+firefly : TvSeries
+firefly =
+    { name = "Firefly"
+    , creator = "Joss Whedon"
+    , episodes = 14
+    }
+```
+
+The type alias also automatically supplies a constructor function for the record.
+This is sometimes surprising since functions in Elm must start with a lowercase character, but type constructors are one exception.
+
+```elm
+firefly : TvSeries
+firefly = TvSeries "Firefly" "Joss Whedon" 14
+```
+
+## Accessing record fields
+
+The main way to [access a field value of a record instance][access] is to use the dot-notation such as `firefly.creator`.
+The Elm compiler also supports special accessor functions starting with a dot `.` such as `.creator` that will work on any record with a field of that name.
+That is the second exception to the rule that functions must start with a lowercase character.
+In fact user-defined functions must start with a lowercase character, but the two examples we just discovered are generated by the compiler, not the programmer.
+
+```elm
+firefly : TvSeries
+firefly =
+    { name = "Firefly"
+    , creator = "Joss Whedon"
+    , episodes = 14
+    }
+
+firefly.name
+    --> "Firefly"
+
+.creator firefly
+    --> "Joss Whedon"
+```
+
+Records can also be _destructured_ in bindings using the [record pattern matching][pattern-matching].
+Destructuring works with any record that has fields of the relevant names.
+
+```elm
+episodesCount : TvSeries -> Int
+episodesCount { episodes } =
+    episodes
+
+complicatedCopy : TvSeries -> TvSeries
+complicatedCopy show =
+    let
+        { name, creator } = show
+    in
+    { name = name
+    , creator = creator
+    , episodes = episodesCount show
+    }
+```
+
+## Updating records
+
+Records are immutable, as everything in Elm, so once a record is defined, its field values can never change.
+There is a [special syntax to create a copy of an existing record, but changing one or more fields][update].
+This syntax uses the pipe symbol `|` to distinguish the original record and the fields that will change, such as `{ oldRecord | field1 = newValue }`.
+
+```elm
+firefly : TvSeries
+firefly =
+    { name = "Firefly"
+    , creator = "Joss Whedon"
+    , episodes = 14
+    }
+
+updatedFirefly : TvSeries
+updatedFirefly =
+    { firefly | creator = "J Whedon", episodes = 15 }
+```
+
+## Comparing records
+
+Elm uses [structural equality][equality], which means that two instances of the same record with identical values are equal.
+
+```elm
+firefly1 = TvSeries "Firefly" "Joss Whedon" 14
+firefly2 = TvSeries "Firefly" "Joss Whedon" 14
+
+firefly1 == firefly2
+    --> True
+```
+
+## Extensible records
+
+Elm also supports [structural typing][structural-typing] meaning that if a function requires a record with an x and y field, it will work with any record that has those fields such as 2D points, 3D points, spaceships, etc.
+Those record types have a pipe `|` in their definition, such as `{ a | x : Float, y : Float }` and are called "extensible records" in Elm terminology.
+Beware of the difference between a pipe `|` in a type definition, which is an extensible record definition (`{ a | x : Int }`), and a pipe `|` an actual record instance which means that we are updating some fields of that record.
+
+```elm
+point2d = { x = 1, y = 2 }
+point3d = { x = 3, y = 4, z = 7 }
+
+a.x point2d --> 1
+.x point3d --> 3
+
+length : { a | x : Float, y : Float } -> Float
+length vector =
+    sqrt (vector.x * vector.x + vector.y * vector.y)
+
+length point2d --> 2.236068
+length point3d --> 5
+
+translateX : { a | x : Float } -> { a | x : Float }
+translateX vector =
+    { vector | x = vector.x + 1 }
+
+translateX point2d --> { x = 2, y = 2 }
+translateX point3d --> { x = 4, y = 4, z = 7 }
+```
+
+[records]: https://elm-lang.org/docs/records
+[record-types]: https://elm-lang.org/docs/records#record-types
+[structural-typing]: https://en.wikipedia.org/wiki/Structural_type_system
+[create]: https://elm-lang.org/docs/records#what-is-a-record-
+[update]: https://elm-lang.org/docs/records#updating-records
+[access]: https://elm-lang.org/docs/records#access
+[pattern-matching]: https://elm-lang.org/docs/records#pattern-matching

concepts/records/links.json

@@ -0,0 +1,18 @@
+[
+  {
+    "url": "https://elm-lang.org/docs/records",
+    "description": "Records"
+  },
+  {
+    "url": "https://elmprogramming.com/record.html",
+    "description": "Records (in more detail)"
+  },
+  {
+    "url": "https://en.wikipedia.org/wiki/Structural_type_system",
+    "description": "Structural types"
+  },
+  {
+    "url": "https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/scopedlabels.pdf",
+    "description": "Extensible records white paper"
+  }
+]