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.
Tillmann
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