Add Semigroupoid type-class

frege/control/Category.fr

@@ -1,31 +1,15 @@
-package frege.control.Category 
-        inline (F.•, F.id)
+package frege.control.Category
+        inline (F.id)
     where
 
-import frege.Prelude hiding (id, .) -- will not be necessary
+import frege.Prelude hiding (id) -- will not be necessary
+import frege.control.Semigroupoid
 
 private type F = (->)
 
---- A class for categories. id and (.) must form a monoid.
-class Category f where
+class Category (Semigroupoid f) => f where
   --- the identity morphism
-  id      :: f a a
-
-  --- morphism composition
-  (.) :: f b c -> f a b -> f a c
-
-infixr 1 `>>>`
-infixr 1 `<<<`
-
---- Right-to-left composition
-(<<<) :: Category f => f b c -> f a b -> f a c
-(<<<) = (.)
-
---- Left-to-right composition
-(>>>) :: Category f => f a b -> f b c -> f a c
-f >>> g = g . f
+  id :: f a a
 
 instance Category (->) where
-  id = \x -> x
-
-  f . g = \a -> f (g a)
+  id = \x -> x

frege/control/Semigroupoid.fr

@@ -0,0 +1,25 @@
+package frege.control.Semigroupoid
+        inline (F.•)
+    where
+
+import frege.Prelude hiding (.)
+
+private type F = (->)
+
+class Semigroupoid f where
+  --- morphism composition
+  (.) :: f b c -> f a b -> f a c
+
+infixr 1 `>>>`
+infixr 1 `<<<`
+
+--- Right-to-left composition
+(<<<) :: Semigroupoid f => f b c -> f a b -> f a c
+(<<<) = (.)
+
+--- Left-to-right composition
+(>>>) :: Semigroupoid f => f a b -> f b c -> f a c
+f >>> g = g . f
+
+instance Semigroupoid (->) where
+  f . g = \a -> f (g a)