[Haskell-beginners] Writing a polymorphic function in Haskell

[Matt Williams]

Dear All,

I am having problems writing a polymorphic function.

I have a type that has two constructors:

data Arg = IndArg {evi::Evi, t1::Treatment, t2::Treatment, out::Outcome,
dir::Direction}
            | MetaArg {target::EviType}
            deriving (Show)

I have a function checks for conflict:

checkConflict :: Arg -> Arg -> Bool
checkConflict a b = if t1 a == t1 b && t2 a == t2 b && dir a /= dir b then
True
                     else False

However, I can't make this work with both types of Argument - if I pass it
MetaArg, it raises an error.

In another language, I would write something like:

checkConflict(ArgA,ArgB):
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