[Haskell-cafe] Trouble with record syntax and classes

Tillmann Rendel rendel at rbg.informatik.tu-darmstadt.de
Mon Feb 26 17:42:18 EST 2007

Hello Thomas,

Thomas Nelson schrieb:
> I'm brand new to haskell and I'm having trouble using classes. The
> basic idea is I want two classes, Sine and MetaSine, that are both
> instances of ISine. This way I can use the act method and recurse
> through the metasines and sines.

That looks too much like object oriented design and too less like 
haskell for me. Have you considered using a standard algebraic type for 
all variantes of sines?

About your code:

> class ISine a where
>     period :: a -> Integer
>     offset :: a -> Integer
>     threshold :: a -> Integer
>     act :: (ISine b) => Integer -> a -> b
>     on :: Integer -> a -> Bool
>     --on needs offset, period, threshold
>     on time self = (mod (time-(offset self)) (period self)) < (threshold self)

on is the same for alle instances? then don't include it in the type 
class, but provide it as polymorphic "helper" function:

on :: (ISine a) => Integer -> a -> Bool
on time self = (mod (time - (offset self)) (period self)) <
                (threshold self)

and don't use self, because it could prevent you from forgetting that 
haskell is not object oriented.

> data Sine =
>     Sine {
>         period :: Integer,
>         offset :: Integer,
>         threshold :: Integer,
>         letter :: String
>         }

You are not allowed to use period, offset and threshold as selector 
names, because they are already used as members of class ISine.

> instance Sine ISine where
>     act time (Sine self)
>         |on time self = [letter self]
>         |otherwise = []

You should provide definitions for all members of class ISine in this 
instance declaration. there are no automatic use of the like-named 
selectors. (in fact, the naming is illegal as pointed out above and by 
the compiler).

Same for MetaSine, of course.

I strongly suggest to ignore type classes and instances for a while and 
start learning about the way haskell represents data as algebraic data 
types. What you tried seems comparable to using a c++ template where a 
simple c function would do. Maybe working through a tutorial before 
trying to implement your own ideas could be helpfull, too.


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