Re: Instances of IArray

[Ketil Z. Malde]

Albert Lai <[EMAIL PROTECTED]> writes:

> I had looked into the source code of IArray a bit.  Like you have
> found, (!) is not a class method. [...]

Right.

Shouldn't they be, though?  I'm probably falling into some
put-everything-in-classes trap, but wouldn't it be nice to overload
these functions (as well as the previously mentioned list operations)?

> Of course, you see the difference when you roll your own IArray
> instance. 

Yes, and given the class members, I'm not at all sure how to implement
it.  My instinct tells me not to mess with it, and invent a different
operator for indexing (which I already did anyway)

-kzm
-- 
If I haven't seen further, it is by standing in the footprints of giants
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Re: Instances of IArray

[Albert Lai]

I had looked into the source code of IArray a bit.  Like you have
found, (!) is not a class method.  In fact, none of the claimed class
methods in the doc is a class method, and no class method of IArray is
documented.  Instead, IArray class methods seem to pertain to
implementation details (I mean, "unsafeAt"!), and user operations
such as (!) are constrained polymorphic functions, e.g.,

(!) :: (IArray a e, Ix ix) => a ix e -> ix -> e

>From the user's point of view, there is little difference between a
class method and a class-constrained polymorphic function when the
provided instances are abstract types.  So the documentation is
telling a white lie.  Of course, you see the difference when you
roll your own IArray instance.

I think the white lie is a balance between exposition simplicity
and implementation tuning.

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Instances of IArray

[Ketil Z. Malde]

Hi,

from the GHC documentation (5.17.1 in the libraries section), I get
the impression that (!) is a member of the IArray class.  While I'm
messing about with kinds and stuff getting this properly instantiated,
I get an error claiming that

EST.lhs:44:
Class `IArray' does not have a method `!'
In the instance declaration for `IArray EST n'

Experimenting with GHCi, I finally worked out

Prelude> :i IArray.IArray
-- ArrayBase.IArray is a class
class (ArrayBase.HasBounds a) => ArrayBase.IArray a :: (*
-> * -> *) e where {
ArrayBase.unsafeArray ::
forall i. (PrelArr.Ix i) => (i, i) -> [(Int, e)] -> a i e;
ArrayBase.unsafeAt ::
forall i. (PrelArr.Ix i) => a i e -> Int -> e;
ArrayBase.unsafeReplace ::
forall i. (PrelArr.Ix i) => a i e -> [(Int, e)] -> a i e
{- has default method -};
ArrayBase.unsafeAccum ::
forall e' i. (PrelArr.Ix i) =>
(e -> e' -> e) -> a i e -> [(Int, e')] -> a i e
{- has default method -};
ArrayBase.unsafeAccumArray ::
forall e' i. (PrelArr.Ix i) =>
(e -> e' -> e) -> e -> (i, i) -> [(Int, e')] -> a i e
{- has default method -};
}

So, unless I'm misinterpreting something, the documentation is
incorrect? 

What I'd like to have, is Int-indexed access to my classes.  ATM,
I've solved it with my own class "Indexed" providing a (?) operator.
This is (obviously) not consistent with the standard types, and in
addition collides slightly with the implicit parameters syntax (works
if I leave a space between the ? and the index)

-kzm
-- 
If I haven't seen further, it is by standing in the footprints of giants
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