I think this is more or less the standard way of doing it. I don't think type classes are the right thing to do. Usually constructors are prefixed with the first character of their type in situations like this (or so I've seen), so you get: data Expression = EValue Value | EVariable Variable | ... data Value = VNumber Number | V... | ... etc... I'm not sure if this answers your question or not, tho... -- Hal Daume III "Computer science is no more about computers | hdaume@isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume On Tue, 8 Oct 2002, Mark T.B. Carroll wrote:

I have a program that basically has,

data Expression = Value Value | EVariable Variable | other stuff ...

data Value = VNumber Number | other stuff ...

data Variable = Variable { variable_name :: String, variable_time :: Expression } data Number = Number { value :: Double, dimension :: Dimension }

newtype VariableCount = VariableCount (Variable, Number)

The VNumber and EVariable constructors are ugly, though, so I was wondering if I should be using typeclasses - e.g.,

class Expression a class Expression a => Value a instance Value Number instance Expression Variable

... but I don't see how to define Variable in such a scheme. Maybe I shouldn't be using typeclasses?

(Obviously, I actually have lots more type constructors in Expression and Value - dyadic expressions, booleans, etc. - the above with just numbers and variables is somewhat truncated, but should suffice.)

-- Mark

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Johan Nordlander wrote:

For comparison, the example could be formulated in O'Haskell as follows:

data Variable = Variable { variable_name :: String, variable_time :: Expression } data Number = Number { value :: Double, dimension :: Dimension }

data Value > Number = other stuff

data Expression > Value, Number = other stuff

newtype VariableCount = VariableCount (Variable, Number)

-- Johan

Ahem, it was brought to my attention that O'Haskell doesn't support Haskell's named datatype fields, which I should be the first to know... I got a little carried away, it seems :-) So I'm rephrasing my example into data Variable = Variable String Expression data Number = Number Double Dimension data Value > Number = other stuff data Expression > Value, Number = other stuff newtype VariableCount = VariableCount (Variable, Number) Apologies, Johan