Use instances to simplify printing code

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

Showable.agda

@@ -0,0 +1,38 @@
+module Showable where
+
+open import Data.String using (String; _++_)
+open import Data.Nat using (ℕ)
+open import Data.Nat.Show using () renaming (show to showNat)
+open import Data.Fin using (Fin)
+open import Data.Fin.Show using () renaming (show to showFin)
+open import Data.Product using (_×_; _,_)
+open import Data.Unit using (⊤; tt)
+
+record Showable {a} (A : Set a) : Set a where
+    field
+        show : A → String
+
+open Showable {{ ... }} public
+
+instance
+    showableString : Showable String
+    showableString = record { show = λ s → "\"" ++ s ++ "\"" }
+
+    showableNat : Showable ℕ
+    showableNat = record { show = showNat }
+
+    showableFin : ∀ {n : ℕ} → Showable (Fin n)
+    showableFin = record { show = showFin }
+
+    showableProd : ∀ {a b} {A : Set a} {B : Set b}
+                     {{ showableA : Showable A }} {{ showableB : Showable B }} →
+                   Showable (A × B)
+    showableProd {{ showableA }} {{ showableB }} = record
+        { show = λ (a , b) →
+            "(" ++ show a ++
+            ", " ++ show b ++
+            ")"
+        }
+
+    showableUnit : Showable ⊤
+    showableUnit = record { show = λ tt → "()" }