Haskell's type system does not directly allow eta expansion, i.e.
instance Map (\x y -> [(x, y)]) a b where
...
is not legal.
The straight-forward way to tackle this is with a newtype declaration, i.e.
newtype Assoc a b = Assoc [(a, b)]
instance Map Assoc a b where
...
works as expected. There's an avenue for you to continue with if you like.
In regards to what GHC was trying to tell you with its error message:
type synonyms (like your Assoc) can never be partially applied. In
your instance, Assoc is not applied to a and b (don't let the adjaceny
fool you), it's just sitting by itself... the most lonely version of
partial application. Hence it croaks.
Hope that helps,
Nick
ps - I've never pondered why it is that type synonyms can't be
partially applied. I'm sure someone will pipe up with the answer.
On 10/28/06, Mathieu Boespflug <[EMAIL PROTECTED]> wrote:
Hi everyone,
I'm running into trouble with type synonyms in instance heads and I
can't figure out what the resulting error message means. The case I'm
considering is as follows:
-- hoist a and b to class variables so that instances declarations may
-- constrain them.
class Map m a b where
toAssoc :: m a b -> [(a, b)]
fromAssoc :: [(a, b)] -> m a b
type Assoc a b = [(a, b)]
instance Map Assoc a b where
toAssoc = id
fromAssoc = id
The class Map is used to allow translation from one map type to
another (FiniteMap, arrays, etc...) by means of expressing the map as
an association list. Useful for defining isomorphisms and so on. Now
I'd like to define an association list as itself a trivial instance of
Map class, but I cannot do so without wrapping the type of an
association list, [(a, b)], behind a type synonym, as I'm not aware of
any way of writing a type constructor of kind (* -> * -> *)
constructing the type [(a, b)].
But when compiling the above code with GHC I get the following error:
Map.hs:9:0:
Type synonym `Assoc' should have 2 arguments, but has been given 0
In the instance declaration for `Map Assoc a b'
Any idea what this means? Also, is there any other way of declaring an
instance for [(a, b)] without using type synonyms?
Many thanks,
Mathieu
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