Add ADTs concept

Author: pwadsworth

concepts/algebraic-data-types/.meta/config.json

@@ -0,0 +1,6 @@
+{
+  "authors": [
+    "pwadsworth"
+  ],
+  "blurb": "Introduction to Algebraic Data Types in Haskell."
+}

concepts/algebraic-data-types/about.md

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

concepts/algebraic-data-types/introduction.md

@@ -0,0 +1,41 @@
+# 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_).
+
+```haskell
+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.
+
+```haskell
+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:
+
+```haskell
+add1 :: Number -> String
+add1 number =
+    case number of
+      NInt    i -> show (i + 1)
+      NFloat  f -> show (f + 1.0)
+      Invalid   -> error "Invalid input"
+```

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

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

concepts/algebraic-data-types/links.json

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

config.json

@@ -1267,6 +1267,11 @@
       "uuid": "ca21a553-6fc1-49be-8f47-730f97330862",
       "slug": "pattern-matching-literals",
       "name": "Pattern Matching Literals"
+    },
+    {
+      "uuid": "cb756cfd-9b5c-408d-bf3f-b2adc8322da3",
+      "slug": "algebraic-data-types",
+      "name": "Algebraic Data Types"
     }
   ],
   "key_features": [