add Functor etc for IfConsistent

lib/Quox/Syntax/DimEq.idr

@@ -29,10 +29,36 @@ data DimEq : Nat -> Type where
 
 
 public export
-data IfConsistent : DimEq d -> a -> Type where
+consistent : DimEq d -> Bool
+consistent ZeroIsOne = False
+consistent (C eqs)   = True
+
+
+public export
+data IfConsistent : DimEq d -> Type -> Type where
   Nothing :      IfConsistent ZeroIsOne a
   Just    : a -> IfConsistent (C eqs)   a
 
+export
+Functor (IfConsistent eqs) where
+  map f Nothing  = Nothing
+  map f (Just x) = Just (f x)
+
+export
+Foldable (IfConsistent eqs) where
+  foldr f z Nothing  = z
+  foldr f z (Just x) = f x z
+
+export
+Traversable (IfConsistent eqs) where
+  traverse f Nothing  = pure Nothing
+  traverse f (Just x) = Just <$> f x
+
+public export
+ifConsistent : Applicative f => (eqs : DimEq d) -> f a -> f (IfConsistent eqs a)
+ifConsistent ZeroIsOne act = pure Nothing
+ifConsistent (C _)     act = Just <$> act
+
 
 export %inline
 zeroEq : DimEq 0