Trouble with record syntax and classes
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. Here's my code: module Main where 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) data Sine = Sine { period :: Integer, offset :: Integer, threshold :: Integer, letter :: String } instance Sine ISine where act time (Sine self) |on time self = [letter self] |otherwise = [] data MetaSine = MetaSine { period :: Integer, offset :: Integer, threshold :: Integer, sines :: (ISine a) => [a] } instance MetaSine ISine where act time (MetaSine self) |on time self = foldr (++) (map (act time) (sines self)) |otherwise = [] The errors I get involve multiple declarations of period, offset, and threshold. Any help would be greatly appreciated. -Thomas
All record fields are in the same namespace, and furthermore this is also the same namespace of functions and class methods. In other words you cannot have two record types containing the same field name, and you cannot have a record field and a function using the same name, and you cannot have a record field and a class method using the same name. You have to choose some other names for the fields of Sine, and yet some other names for the fields of MetaSine. In "instance Sine ISine where ...", you must implement the methods "period", "offset", and "threshold", not just "act". Similarly for "instance MetaSine ISine where ...". The implementations of method "act" are syntactically wrong as well as semantically wrong. I do not know what is a right implementation. Actually given the overly general signature act :: (ISine b) => Integer -> a -> b I do not think there is any possible implementation at all. I submit that you have set a goal too ambitious and too magical.
On 2/26/07, Thomas Nelson
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.
'class' in Haskell doesn't mean the same as 'class' in C++ or Java. I found it easier at first to thing of them as: A Haskell 'class' is more like a Java interface. Haskell types are more like what you might think of as 'class'es. Haskell 'instance' means Java 'implement' There is no word that means that same as 'instance' from Java/C++ terminology. I suppose we would call them 'values' or something. Somebody more knowledgeable can describe the etymology of the terms, but these 3 observations should help.
data Sine = Sine { period :: Integer, offset :: Integer, threshold :: Integer, letter :: String}
instance Sine ISine where act time (Sine self) |on time self = [letter self] |otherwise = []
To be honest, I'm not sure what you're trying to do here, so beware of my advice... You might want to do this instead: data Sine = Sine Integer Integer Integer String instance ISine Sine where -- note that ISine should come before Sine period (Sine p _ _ _ _) = p period (Sine _ o _ _ _) = o -- and so on ... There can only be a single function called period, which will take a thing of any type which is an instance of ISine and return an Integer. So every time you tell Haskell "this type is to be an implementation of ISine" you have to write the period function for it as I have done for Sine here.
-Thomas
Aaron
G'day all.
Quoting Aaron McDaid
'class' in Haskell doesn't mean the same as 'class' in C++ or Java. I found it easier at first to thing of them as: A Haskell 'class' is more like a Java interface. Haskell types are more like what you might think of as 'class'es. Haskell 'instance' means Java 'implement' There is no word that means that same as 'instance' from Java/C++ terminology. I suppose we would call them 'values' or something. Somebody more knowledgeable can describe the etymology of the terms, but these 3 observations should help.
When you type "class Foo" in Java or C++, it does three things: 1. It declares a new type called "Foo". 2. It declares a _set_ of types (i.e. a "class"). 3. It declares that the type Foo (and all of its subtypes) is a member of the set of types Foo. In Haskell, these three operations are distinct. 1. You declare a new type using "data" or "newtype". 2. You declare a new set of types using "class". 3. You declare that a type is a member of a class using "instance". Cheers, Andrew Bromage
G'day all.
Oh, one more thing.
Quoting Aaron McDaid
Somebody more knowledgeable can describe the etymology of the terms, [...]
You can think of a type as a set of values. For example, Bool is the set { False, True }. A "class", then, is a set of types. The distinction between "set" and "class" comes from the various set theories (Goedel-Bernays-von Neumann set theory being the most common) which try to avoid Russell's Paradox. For those who are don't know about Russell's Paradox, take a look at the Wikipedia entry before going on: http://en.wikipedia.org/wiki/Russell%27s_paradox The idea behind GBN set theory is to distinguish between "sets", which are always well-behaved, and "classes", which are not necessarily so well-behaved. Russell's Paradox is resolved by setting up your axioms such that the paradoxical "set of all sets with property X" is not, itself, a set, but a class. By analogy, we call a set of types, or a "set of sets", a "class". Cheers, Andrew Bromage
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
On Mon, Feb 26, 2007 at 01:22:57PM -0600, Thomas Nelson wrote:
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. Here's my code:
module Main where
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)
data Sine = Sine { period :: Integer, offset :: Integer, threshold :: Integer, letter :: String }
instance Sine ISine where The other way round: instance class type where
Your errors have nothing to do with classes. Just try module Main where data MetaSine = MetaSine {period :: Integer } data Sine = Sine { period :: Integer } main = print "test" which won't compile and show the same error Solution? 1) single module solution: a) using records: Use prefixes data MetaSine = MetaSine {msPeriod :: Integer } data Sine = Sine { sPeriod :: Integer } b) using the same accessor function "period" requires classes I think this is what you had in mind when introducing ISine Of course you can add this to a), too. data MetaSine = MetaSine Integer data Sine = Sine sPeriod class Period where period :: Integer instance Period Sine where period (Sine p) = p instance Period MetaSine where period (MetaSine p) = p 2) use for each datatype another module. Then you can define record name period more than once. But you have to access them using ModuleSine.period or ModuleMetaSine.period which is not what you want I think. HTH Marc
participants (6)
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Aaron McDaid -
ajb＠spamcop.net -
Albert Y. C. Lai -
Marc Weber -
Thomas Nelson -
Tillmann Rendel