1 files changed, 12 insertions, 0 deletions
diff --git a/Haskell-book/15/optional/src/Optional.hs b/Haskell-book/15/optional/src/Optional.hs
new file mode 100644
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+++ b/Haskell-book/15/optional/src/Optional.hs
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+module Optional where
+
+data Optional a =
+    Nada
+  | Only a
+  deriving (Eq, Show)
+
+instance Monoid a => Monoid (Optional a) where
+    mempty = Nada
+    mappend x Nada = x
+    mappend Nada x = x
+    mappend (Only x) (Only y) = Only (mappend x y)