Add Homogeneous and HomogeneousRowList for homogeneous rows (#20)

* Add RecordOf and FieldOf for homogeneous records

* Rename to `Homogenous` and move to `Row` type

* Add `TypeEquals` constraint on field type
* Add funcitonal dependency for `FieldOf`

* Add comment explaining Homogeneous class

* Add missin fun dep

* Add Type.Row.Homogeneous

* Remove unwanted comment

That was incorrectly copied by the PureScript Emacs layer.

* Rename `FieldOf` to `HomogenousRowList`

To better communicate which class should be used in regular code.

* Rename instances to coincide previous renaming

Author: paulyoung

src/Type/Row/Homogeneous.purs

@@ -0,0 +1,21 @@
+module Type.Row.Homogeneous
+  ( class Homogeneous
+  , class HomogenousRowList
+  ) where
+
+import Type.Equality (class TypeEquals)
+import Type.Row (class RowToList, Cons, Nil, kind RowList)
+
+-- | Ensure that every field in a row has the same type.
+class Homogeneous (row :: # Type) fieldType | row -> fieldType
+instance homogeneous
+  :: ( RowToList row fields
+     , HomogenousRowList fields fieldType )
+  => Homogeneous row fieldType
+
+class HomogenousRowList (rowList :: RowList) fieldType | rowList -> fieldType
+instance homogenousRowListCons
+  :: ( HomogenousRowList tail fieldType
+     , TypeEquals fieldType fieldType2 )
+  => HomogenousRowList (Cons symbol fieldType tail) fieldType2
+instance homogenousRowListNil :: HomogenousRowList Nil fieldType