Complete daniel-chambers' PR adding Show instances to generics rep types (purescript#250)

* Add Show instance to generic rep types (complete daniel-chambers PR)

Helps one see the generic representation for a given value

* Add `@` in front of constructor name in Show instance for generic type

* Add tests for Inl, NoArguments, and Product

Author: JordanMartinez

src/Data/Generic/Rep.purs

@@ -11,6 +11,9 @@ module Data.Generic.Rep
   , Argument(..)
   ) where
 
+import Data.Semigroup ((<>))
+import Data.Show (class Show, show)
+import Data.Symbol (class IsSymbol, reflectSymbol)
 import Type.Proxy (Proxy(..))
 
 -- | A representation for types with no constructors.
@@ -19,19 +22,35 @@ data NoConstructors
 -- | A representation for constructors with no arguments.
 data NoArguments = NoArguments
 
+instance showNoArguments :: Show NoArguments where
+  show _ = "NoArguments"
+
 -- | A representation for types with multiple constructors.
 data Sum a b = Inl a | Inr b
 
+instance showSum :: (Show a, Show b) => Show (Sum a b) where
+  show (Inl a) = "(Inl " <> show a <> ")"
+  show (Inr b) = "(Inr " <> show b <> ")"
+
 -- | A representation for constructors with multiple fields.
 data Product a b = Product a b
 
+instance showProduct :: (Show a, Show b) => Show (Product a b) where
+  show (Product a b) = "(Product " <> show a <> " " <> show b <> ")"
+
 -- | A representation for constructors which includes the data constructor name
 -- | as a type-level string.
 newtype Constructor (name :: Symbol) a = Constructor a
 
+instance showConstructor :: (IsSymbol name, Show a) => Show (Constructor name a) where
+  show (Constructor a) = "(Constructor @" <> show (reflectSymbol (Proxy :: Proxy name)) <> " " <> show a <> ")"
+
 -- | A representation for an argument in a data constructor.
 newtype Argument a = Argument a
 
+instance showArgument :: Show a => Show (Argument a) where
+  show (Argument a) = "(Argument " <> show a <> ")"
+
 -- | The `Generic` class asserts the existence of a type function from types
 -- | to their representations using the type constructors defined in this module.
 class Generic a rep | a -> rep where

test/Data/Generic/Rep.purs

@@ -123,6 +123,15 @@ testGenericRep = do
   assert "Checking show" $
     show (cons 1 (cons 2 Nil)) == "(Cons { head: 1, tail: (Cons { head: 2, tail: Nil }) })"
 
+  assert "Checking show for generic types: Inl, NoArguments" $
+    show (G.from (Nil :: List Int)) == "(Inl (Constructor @\"Nil\" NoArguments))"
+
+  assert "Checking show for generic types: Inr, Constructor, and Single Argument" $
+    show (G.from $ cons 1 Nil) == "(Inr (Constructor @\"Cons\" (Argument { head: 1, tail: Nil })))"
+
+  assert "Checking show for generic types: Product" $
+    show (G.from $ Pair 1 2) == "(Constructor @\"Pair\" (Product (Argument 1) (Argument 2)))"
+
   assert "Checking equality" $
     cons 1 (cons 2 Nil) == cons 1 (cons 2 Nil)