I want to implement a simple one-to-many data structure in Haskell. Each key of type k associated with set of elements of type v.

{-# LANGUAGE AllowAmbiguousTypes #-}

module SetMap where
  
import Data.Map
import Data.Set

class SetMap k v m where
  add :: k -> v -> m -> m -- add value to corresponding set
  delete :: k -> m -> m   -- delete all values associated with this key   
  get :: k -> m -> [v]    -- get values associated with this key
  
type SetMapImpl k v = Map k (Set v)  

instance SetMap k v (SetMapImpl k v) where -- duplication here!
  ...

m here is a type of implementation. But it is also parametrized with k and v. Any ways how can I declare it? Or is it OK to do as shown above?

I expect something like this but it does not compile.

class SetMap k v m where
  add :: k -> v -> m k v -> m k v-- add value to corresponding set
  delete :: k -> m k v -> m k v  -- delete all values associated with this key   
  get :: k -> m k v -> [v]    -- get values associated with this key
  
type SetMapImpl k v = Map k (Set v)  

instance SetMap k v SetMapImpl where
  ...

CodePudding user response：

I think it's easier if you exploit type families. You'll probably need to enable a few extensions.

class SetMap m where
  type Key m
  type Val m
  add :: Key m -> Val m -> m -> m
  delete :: Key m -> m -> m
  get :: Key m -> m -> [Val m]

instance (Ord k, Ord v) => SetMap (Map k (Set v)) where
   type Key (Map k (Set v)) = k
   type Val (Map k (Set v)) = v
   add k v m = ...
   ...

I don't know if using a type class here is a good idea. If you have multiple types that are instances of that class, it might be worth it. Maybe. Be careful not to overgeneralize.

CodePudding user response：

You have several options:

0. Classless

First consider why you need a class at all. Why not just implement those methods right away as concrete functions?

type SetMapImpl k v = Map k (Set v)  -- In fact I would tend to call this
                                     -- simply `SetMap` in this version.

add :: k -> v -> SetMapImpl k v -> SetMapImpl k v
add = ...

delete :: k -> SetMapImpl k v -> SetMapImpl k v
delete = ...

...

Notice that you don't need a newtype in this case, because SetMapImpl is only ever mentioned with both arguments fully applied. But it might still be a good idea to make it a newtype, for clearer encapsulation, error messages etc..

Obviously, this approach means you can't write code that's polymorphic over different implementations, but unless you have a good reason to write such code it is best not to worry about this. Keep it simple. Thanks to Haskell's strong type system it is still easy to generalize your code later on, should that become necessary.

1. Class for parameterised types

If you want the methods to look like add :: k -> v -> m k v -> m k v, then k and v are parameters of the type and of the methods, but not of the abstraction that's expressed by the class. Hence this should rather look like this:

class SetMap m where
  add :: k -> v -> m k v -> m k v

In this version, you must make the SetMapImpl a newtype or data, because m is mentioned without parameters in the class head. That's not much of a problem though. What is a problem is that the add method would now have to work with any types k and v, which it can't: for Set you need the Ord constraint. There are a couple of ways this can be achieved:

1a. Hard-coded constraints

class SetMap m where
  add :: (Ord k, Ord v) => k -> v -> m k v -> m k v

newtype SetMapImpl k v = ...
instance SetMap SetMapImpl where
  add = ...

Simple, but inflexible. In particular, a main motivation for having a class at all is that you could have other implementations using other constraints like Hashable; this approach does not support that.

1b. User-selectable constraints

{-# LANGUAGE TypeFamilies, ConstraintKinds #-}
import Data.Kind (Constraint)

class SetMap m where
  type KeyConstraint m k :: Constraint
  type ValueConstraint m v :: Constraint
  add :: (KeyConstraint m k, ValueConstraint m v)
             => k -> v -> m k v -> m k v

newtype SetMapImpl k v = ...
instance SetMap SetMapImpl where
  type KeyConstraint SetMapImpl k = Ord k
  type ValueConstraint SetMapImpl v = Ord v
  add = ...

I rather like this approach, because it expresses that the SetMap-implementations are parametric, but still allows imposing any required constraint on the contained types. It is a bit complicated to get this right, though, and easy to get confused with the different abstract constraints.

1c. Wrap the constraints into the value level

Since all your methods take one already existing set-map as the argument, and that set-map would already need the knowledge that the keys and values are Ord (or Hashable) to be constructed in the first place. So you can in fact get away without any constraints on the method. But you do need to wrap the constraints in the type itself.

{-# LANGUAGE GADTs #-}

class SetMap m where
  add :: k -> v -> m k v -> m k v

data SetMapImpl k v where
  SetMapImpl :: (Ord k, Ord v) => Map k (Set v) -> SetMapImpl k v

instance SetMap SetMapImpl where
  add k v (SetMapImpl m) = ...

And now in the ... you will have the Ord k and Ord v constraints available though the class knows nothing about them.

I would rather not recommend this approach though. It tends to become awkward having to pass the constraints at the value level, and for generically creating new set-maps you'll need them on the type level anyway.

2. A class, but without parametricity on the set-maps

See chi's answer. This is what I would probably go with. Although it is arguably less elegant that the class doesn't have m in parameterised form at all, this is not really a restriction (since the instance can still be polymorphic over all appropriate key- and value types, and you can simply state the constraints right there). This approach is more explicit, and in my experience it tends to be a lot clearer what you're doing when the key- and value types are actually called Key m and Value m. This does come with some verbosity penalty, but it's probably worth it.

Another advantage is that this can easily deal with implementations that really are not parametric at all; e.g. IntMap only allows keys of type Int.

CodePudding user response：

Your last code example doesn't compile because type aliases like SetMapImpl need to be fully applied to their arguments before you can use them in an instance head. If you add a newtype wrapper to make SetMapImpl an actual type instead of just an alias, the following will compile fine:

class SetMap k v m where
  add :: k -> v -> m k v -> m k v
  delete :: k -> m k v -> m k v
  get :: k -> m k v -> [v]

newtype SetMapImpl k v = SetMapImpl (Map k (Set v))

instance (Ord k, Ord v) => SetMap k v SetMapImpl where
  add k v (SetMapImpl s) = SetMapImpl $ ...
  delete k (SetMapImpl s) = SetMapImpl $ ...
  get k (SetMapImpl s) = ...

The instance methods will require a little wrapping and unwrapping boilerplate as shown, but it will be cost-free in the compiled code.