Thomas Davie wrote:
Hi,
I've met an interesting problem in terms of how to type a data
structure and the functions that operate upon it.
The problem centres around a single data type. This data type can be
constructed in multiple ways using different functions, depending on
the options the user specifies. That's all simple enough. The
problem really comes later on. Depending on the function used
generate the data structure I want to use different functions later
on for example, to display the data.
Thus I have a typical classes problem, in that I have several
implementations of essentially the same function for different
circumstances. The problem is, they must all operate on the same
data type, so I cannot define them as seperate instances.
Anyone got any ideas how to type this?
You could add a field to the data type to store a record of closures to
operate on it eg
data T = T {_ops :: Ops, ...}
data Ops = Ops{_print :: IO ()}
print :: T -> IO ()
print = _print . _ops
so the different smart constructors for T would fill in the Ops member as
required - in other words you can make your own dictionary explicitly to get
more flexibility instead of using the instance/class mechanism.
Regards, Brian.
--
Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.
http://www.metamilk.com
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