add compose

record.cabal

@@ -1,5 +1,5 @@
 name:                record
-version:             0.1.0.50
+version:             0.1.0.52
 synopsis:            Efficient, type safe records implemented using GADTs and type level strings.
 homepage:            http://quasimal.com/projects/records
 license:             BSD3

src/Data/Record.hs

@@ -31,6 +31,9 @@ module Data.Record ( key
                    , alter
                    , append
                    , Record
+                   , compose
+                   , decompose
+                   , (:.)
                    , P
                    , (:=)
                    , type (++)
@@ -39,6 +42,8 @@ module Data.Record ( key
 import Language.Haskell.TH.Syntax
 import Language.Haskell.TH.Quote
 import Language.Haskell.TH.Lib
+import Control.Category ((.))
+import Prelude hiding ((.))
 
 -- | A key of a record. This does not exist at runtime, and as a tradeoff,
 -- you can't do field access from a string and a Typeable context, although
@@ -74,6 +79,12 @@ type family Wrap (w :: a) x
 type instance Wrap (w :: * -> *) x = w x
 type instance Wrap P x = x
 
+-- | Gross
+newtype (w :. m) x = Wmx { decompose :: w (m x) }
+
+compose ::  (a -> w (m x)) -> a -> (w :. m) x
+compose f x = Wmx (f x)
+
 data Record w r where 
     C :: Wrap w e -> Record w r -> Record w (k := e ': r)
     E :: Record w '[]

src/Example.hs

@@ -1,2 +1,19 @@
 {-# LANGUAGE TypeOperators, DataKinds, QuasiQuotes #-}
+import Data.Record
+import Data.IORef
+
+type Point 
+ = '[ "x"       := Double
+    , "y"       := Double
+    , "z"       := Double 
+    , "colour"  := (Int, Int, Int) ]
+
+op :: Record P Point
+op = 0 & 0 & 0 & (0,0,0) & end
+
+main :: IO ()
+main = do
+    let p = box (compose newIORef) op
+    print "yes"
+