X-Git-Url: https://git.cielonegro.org/gitweb.cgi?a=blobdiff_plain;f=Types%2FData%2FMaybe.hs;fp=Types%2FData%2FMaybe.hs;h=997c93ba77ec81afd3491761e55354ed7b77d387;hb=66d175582375d19adfa8747c9e1c468138d47583;hp=0000000000000000000000000000000000000000;hpb=000307857df5266907964aff4ecc9e118314fe3f;p=hs-rrdtool.git

diff --git a/Types/Data/Maybe.hs b/Types/Data/Maybe.hs
new file mode 100644
index 0000000..997c93b
--- /dev/null
+++ b/Types/Data/Maybe.hs
@@ -0,0 +1,42 @@
+{-# LANGUAGE
+  EmptyDataDecls,
+  TypeFamilies
+  #-}
+module Types.Data.Maybe
+    ( Maybe
+    , Nothing
+    , Just
+
+    , IsNothing
+    , IsJust
+
+    , FromJust
+    , FromMaybe
+    )
+    where
+
+import Prelude ()
+import Types.Data.Bool
+
+
+data Nothing
+data Just a
+
+class    Maybe a
+instance Maybe Nothing
+instance Maybe (Just a)
+
+type family   IsNothing m
+type instance IsNothing Nothing  = True
+type instance IsNothing (Just a) = False
+
+type family   IsJust m
+type instance IsJust Nothing  = False
+type instance IsJust (Just a) = True
+
+type family   FromJust m
+type instance FromJust (Just a) = a
+
+type family   FromMaybe a m
+type instance FromMaybe a Nothing   = a
+type instance FromMaybe a (Just a') = a'