Re: [Haskell-beginners] Writing a polymorphic function in Haskell

Dear All, Thanks so much for your help. It was largely a syntactic issue around pattern matching - but I also learnt about the @ operator for patter-matching. The correct code is posted below in case it helps anyone else. The only other think would be to rewrite eviCheck as an inline function using where, but I am happy with this for now. Thanks for your speedy help. BW, Matt checkConflict3 :: Arg -> Arg -> Bool checkConflict3 (IndArg _ at1 at2 _ adir) (IndArg _ bt1 bt2 _ bdir) = if at1 == bt1 && at2 == bt2 && adir /= bdir then True else if at1 == bt2 && at2 == bt1 && revDir (adir) /= bdir then True else False checkConflict3 (IndArg e _ _ _ _) (MetaArg a) = eviCheck e a checkConflict3 a@(MetaArg a) b@(IndArg e _ _ _ _) = eviCheck b a checkConflict3 (MetaArg _) (MetaArg _) = False checkConflict3 _ _ = False --A final catchall eviCheck:: Evi -> EviType -> Bool eviCheck (SimpEvi _ _ _ _ type1) type2 = if type1 == type2 then True else False On 22 September 2017 at 21:16, David McBride wrote:

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.

...

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

On Fri, Sep 22, 2017 at 3:53 PM, Matt Williams wrote: 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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