Moved step into its own module

3 files changed, 77 insertions, 33 deletions

diff --git a/Spear.cabal b/Spear.cabal
index ea5eafc..a19d89f 100644
--- a/Spear.cabal
+++ b/Spear.cabal
@@ -61,6 +61,7 @@ library
                      Spear.Scene.Graph

                      Spear.Scene.Loader

                      Spear.Scene.SceneResources

+                     Spear.Step

                      Spear.Sys.Store

                      Spear.Sys.Store.ID

                      Spear.Sys.Timer

diff --git a/Spear/Step.hs b/Spear/Step.hs
new file mode 100644
index 0000000..5df873d
--- /dev/null
+++ b/Spear/Step.hs
@@ -0,0 +1,75 @@
+{-# LANGUAGE FlexibleInstances #-}
+module Spear.Step
+(
+    -- * Definitions
+    Step(..)
+,   Elapsed
+,   Dt
+    -- * Constructors
+,   sid
+,   spure
+,   sfst
+,   ssnd
+    -- * Combinators
+,   (.>)
+,   (<.)
+,   szip
+)
+where
+
+import Data.Monoid
+
+type Elapsed = Double
+type Dt = Float
+
+-- | A step function.
+data Step a b = Step { step :: Elapsed -> Dt -> a -> (b, Step a b) }
+
+-- | Step identity.
+sid :: Step a a
+sid = Step $ \_ _ a -> (a, sid)
+
+-- | The step that returns the first component in the tuple.
+sfst :: Step (a,b) a
+sfst = spure $ \(a,_) -> a
+
+-- | The step that returns the second component in the tuple.
+ssnd :: Step (a,b) b
+ssnd = spure $ \(_,b) -> b
+
+-- | Construct a step from a pure function.
+spure :: (a -> b) -> Step a b
+spure f = Step $ \_ _ x -> (f x, spure f)
+
+instance Functor (Step a) where
+         fmap f (Step s1) = Step $ \elapsed dt x ->
+              let (a, s') = s1 elapsed dt x
+              in (f a, fmap f s')
+
+instance Monoid (Step a a) where
+         mempty = sid
+
+         mappend (Step s1) (Step s2) = Step $ \elapsed dt a ->
+                 let (b, s1') = s1 elapsed dt a
+                     (c, s2') = s2 elapsed dt b
+                 in (c, mappend s1' s2')
+
+-- Combinators
+
+-- | Chain two steps.
+(.>) :: Step a b -> Step b c -> Step a c
+(Step s1) .> (Step s2) = Step $ \elapsed dt a ->
+      let (b, s1') = s1 elapsed dt a
+          (c, s2') = s2 elapsed dt b
+      in (c, s1' .> s2')
+
+-- | Chain two steps.
+(<.) :: Step a b -> Step c a -> Step c b
+(<.) = flip (.>)
+
+-- | Evaluate two steps and zip their results.
+szip :: (a -> b -> c) -> Step d a -> Step d b -> Step d c
+szip f (Step s1) (Step s2) = Step $ \elapsed dt d ->
+     let (a, s1') = s1 elapsed dt d
+         (b, s2') = s2 elapsed dt d
+     in (f a b, szip f s1' s2')
\ No newline at end of file
diff --git a/demos/pong/Pong.hs b/demos/pong/Pong.hs
index 9a3138b..b323aa2 100644
--- a/demos/pong/Pong.hs
+++ b/demos/pong/Pong.hs
@@ -11,44 +11,12 @@ where
 import Spear.Math.AABB
 import Spear.Math.Spatial2
 import Spear.Math.Vector
+import Spear.Step
 
 import Data.List (foldl')
 import Data.Monoid
 import GHC.Float (double2Float)
 
-type Elapsed = Double
-type Dt = Float
-
--- Step function
-
-data Step a b = Step { step :: Elapsed -> Dt -> a -> (b, Step a b) }
-
-sid :: Step a a
-sid = Step $ \_ _ a -> (a, sid)
-
-spure :: (a -> b) -> Step a b
-spure f = Step $ \_ _ x -> (f x, spure f)
-
-smap :: (a -> b) -> Step c a -> Step c b
-smap f (Step s1) = Step $ \elapsed dt x ->
-     let (a, s') = s1 elapsed dt x
-     in (f a, smap f s')
-
-(.>) :: Step a b -> Step b c -> Step a c
-(Step s1) .> (Step s2) = Step $ \elapsed dt a ->
-      let (b, s1') = s1 elapsed dt a
-          (c, s2') = s2 elapsed dt b
-      in (c, s1' .> s2')
-
-(.<) :: Step a b -> Step c a -> Step c b
-(.<) = flip (.>)
-
-sfst :: Step (a,b) a
-sfst = spure $ \(a,_) -> a
-
-ssnd :: Step (a,b) b
-ssnd = spure $ \(_,b) -> b
-
 -- Game events
 
 data GameEvent