Add all the A tasks

Task/Abstract-type/Agda/abstract-type.agda

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+module AbstractInterfaceExample where
+
+open import Function
+open import Data.Bool
+open import Data.String
+
+-- * One-parameter interface for the type `a' with only one method.
+
+record VoiceInterface (a : Set) : Set where
+  constructor voice-interface
+  field say-method-of : a → String
+
+open VoiceInterface
+
+-- * An overloaded method.
+
+say : {a : Set} → ⦃ _ : VoiceInterface a ⦄ → a → String
+say ⦃ instance ⦄ = say-method-of instance
+
+-- * Some data types.
+
+data Cat : Set where
+  cat : Bool → Cat
+
+crazy! = true
+plain-cat = false
+
+-- | This cat is crazy?
+crazy? : Cat → Bool
+crazy? (cat x) = x
+
+-- | A 'plain' dog.
+data Dog : Set where
+  dog : Dog
+
+-- * Implementation of the interface (and method).
+
+instance-for-cat : VoiceInterface Cat
+instance-for-cat = voice-interface case where
+  case : Cat → String
+  case x with crazy? x
+  ... | true = "meeeoooowwwww!!!"
+  ... | false = "meow!"
+
+instance-for-dog : VoiceInterface Dog
+instance-for-dog = voice-interface $ const "woof!"
+
+-- * and then:
+--
+-- say dog => "woof!"
+-- say (cat crazy!) => "meeeoooowwwww!!!"
+-- say (cat plain-cat) => "meow!"
+--