It is a mistake to use record syntax with data types with multiple constructors. It creates a partial function that dies when you try to use it on a constructor that does not have that field. It is a flaw in the language that many people would like to see removed because it allows programs to compile that will fail at run time, possibly unexpectedly. Arg can have a constructor that does not have an evi. Also what happens if you compare an IndArg to a MetaArg? Or a MetaArg to another MetaArg? You should remove all of these record names, and then write checkConflict as follows (warning untested): checkConflict :: Arg -> Arg -> Bool checkConflict (IndArg _ at1 at2 _ adir) (IndArg _ bt1 bt2 _ bdir) = at1 == bt1 && at2 == bt2 && adir /= bdir -- checkConflict (IndArg ...) (MetaArg) = ??? -- checkConflict a@(MetaArg ...) b@(IndArg ...) = checkConflict b a -- checkConflict (MetaArg _) (MetaArg _) = ??? checkConflict _ _ -> False -- perhaps a catchall? However, you may keep the records as long as you are absolutely sure you are using them only in places where you have the appropriate constructor. On Fri, Sep 22, 2017 at 3:53 PM, Matt Williams <matt.williams45.mw@gmail.com> wrote:
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):
_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners