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Re: Writing a polymorphic function in Haskell (David McBride) ---------------------------------------------------------------------- Message: 1 Date: Fri, 22 Sep 2017 20:53:55 +0100 From: Matt Williams <matt.williams45...@gmail.com> To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners@haskell.org> Subject: [Haskell-beginners] Writing a polymorphic function in Haskell Message-ID: <caftvgqz8ei80gfm5qtnjgnmkbk3lfhqsfa67gcefs4roo95...@mail.gmail.com> Content-Type: text/plain; charset="utf-8" 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): -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.haskell.org/pipermail/beginners/attachments/20170922/f2c55e22/attachment-0001.html> ------------------------------ Message: 2 Date: Fri, 22 Sep 2017 20:55:43 +0100 From: Matt Williams <matt.williams45...@gmail.com> To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners@haskell.org> Subject: Re: [Haskell-beginners] Writing a polymorphic function in Haskell Message-ID: <caftvgqzs4isjfnpz1febi1qjxozchsqwqpfutjopmecucuh...@mail.gmail.com> Content-Type: text/plain; charset="utf-8" Apologies - sent too soon - code edited below 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): > if type(ArgA) == IndArg: <do this> elif type(ArgA) == MetaArg: <do other> Any thoughts would be welcomed. BW, Matt -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.haskell.org/pipermail/beginners/attachments/20170922/e57fae1d/attachment-0001.html> ------------------------------ Message: 3 Date: Fri, 22 Sep 2017 15:14:29 -0500 From: aditya siram <aditya.si...@gmail.com> To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners@haskell.org> Subject: Re: [Haskell-beginners] Writing a polymorphic function in Haskell Message-ID: <CAJrReyiVqJKM=bxka_zuojombvchzyqsdupxthdw0wvxtl3...@mail.gmail.com> Content-Type: text/plain; charset="utf-8" Not tested but maybe adding an Eq instance to your datatype would help? So try changing 'deriving (Show)' to 'deriving (Show, Eq)'. On Fri, Sep 22, 2017 at 2:55 PM, Matt Williams <matt.williams45...@gmail.com > wrote: > Apologies - sent too soon - code edited below > > 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): >> > if type(ArgA) == IndArg: > <do this> > elif type(ArgA) == MetaArg: > <do other> > > Any thoughts would be welcomed. > > BW, > Matt > > > > _______________________________________________ > Beginners mailing list > Beginners@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.haskell.org/pipermail/beginners/attachments/20170922/5e69bd53/attachment-0001.html> ------------------------------ Message: 4 Date: Fri, 22 Sep 2017 16:16:55 -0400 From: David McBride <toa...@gmail.com> To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners@haskell.org> Subject: Re: [Haskell-beginners] Writing a polymorphic function in Haskell Message-ID: <can+tr40gkzbxxxhfrmnojyqaxxf0__jfzveqjhl2ukymcwy...@mail.gmail.com> Content-Type: text/plain; charset="UTF-8" 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...@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 > ------------------------------ Subject: Digest Footer _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners ------------------------------ End of Beginners Digest, Vol 111, Issue 14 ******************************************