On 2/28/11 2:43 AM, Yitzchak Gale wrote:
You have written a large software system in Haskell. Wishing to play to Haskell's strength, you have structured your system as a series of composable layers. So you have data types
Layer1, Layer2, ...
and functions
layer2 :: Layer1 -> Layer2 layer3 :: Layer2 -> Layer3 ...
etc.
Assuming you actually name them Layer1, Layer2, etc, or use any other regular naming scheme, you can break apart the names and use typeclasses to help out: type family Layer :: * -> * data Z data S n class Layerable n where layer :: Layer n -> Layer (S n) Then it's just a matter of getting the right number of them, a la lifting through monad transformer stacks. Of course, from here it's not that hard to add in some type hackery to do the lifting for you, a la "Data types a la Carte"[1]. It's not the cleanest thing ---there's a good deal of boilerplate up front--- but once it's set up, it should Just Work(tm). [1] http://www.cs.nott.ac.uk/~wss/Publications/DataTypesALaCarte.pdf -- Live well, ~wren