On Fri, 20 Apr 2007, Justin Bailey wrote:
Here's the trick I thought was neat. When creating the MassTime value, I capture it in a closure and bind it to the selector function, as below:
makeMass :: Day -> MassTime makeMass day = mass where -- Use trick here to capture mass defined and pass to our selection function. mass = MassTime ... (weekendMassSelector mass) ...
'weekendMassSelector' is defined elsewhere, and basically knows how to count up the right number of participants based on the mass definition given. It's signature is:
weekendMassSelector :: MassTime -> Participants -> Maybe Participants
So weekendMassSelector gets the MassTime value it needs, before it is even constructed. Partial evaluation then gives a function with the (Participants -> Maybe Participants) signature required by the data constructor. I love Haskell - this just seems too cool.
I'd call that http://www.haskell.org/hawiki/TyingTheKnot However, I wonder if it is clearer to use 'let' for holding the computed weekendMass value before passing it to MassTime and somewhere else: let wm = computeWM in MassTime {otherData = f wm, weekendMassSelector = wm, ...} In an imperative language you would certainly write mass.weekend := bla; mass.otherData := f(mass.weekend);