Writing a polymorphic function in Haskell

Matt Williams

22 Sep 2017 22 Sep '17

3:53 p.m.

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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Matt Williams

22 Sep 22 Sep

3:55 p.m.

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

aditya siram

4:14 p.m.

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

...

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

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David McBride

4:16 p.m.

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 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):

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Matt Williams

23 Sep 23 Sep

11:17 a.m.

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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aditya siram
David McBride
Matt Williams