[Haskell-cafe] Data.Foldable UArray

Marcus D. Gabriel marcus at gabriel.name
Fri Feb 21 21:27:21 UTC 2014

Yes, Tom it does help.  I will not cogitate on this in the morning.

- Marcus

On 21/02/2014 22:14, Tom Ellis wrote:
> On Fri, Feb 21, 2014 at 09:30:12PM +0100, Marcus D. Gabriel wrote:
>>> Prelude> :k Data.Array.Unboxed.UArrays
>>> Data.Array.Unboxed.UArray :: * -> * -> *
>> which is why I tried my first naive solution.
> Hi Marcus,
> This means that 'UArray' is a type constructor of two arguments, i.e. for
> types 'i' and 'e'
>     UArray i e
> is a type.  It is an array that is indexed by elements of type 'i' (which
> would typically be something like 'Int' in practice).  It is also declaring
> that ostensibly it contains unboxed values of type 'e' (which in practice
> could be 'Bool', 'Char', 'Int', 'Double' and many other things besides). 
> For example, a type of an unboxed array you might use could be
>     UArray Int Double
> an array of doubles indexed by ints.  However if you look at the definition
> of 'UArray'
>     http://hackage.haskell.org/package/array-
> you will see that the 'e' type parameter is not actually used in the
> definition of the datatype!  It is a so-called "phantom type".  The datatype
> doesn't "actually" contain any 'e's.  Instead it contains a ByteString into
> which certain 'e's can be written in a way which is in a sense "unsafe" but
> only a "safe" interface is provided.  (See below).
>> I note that we have also
>>> Prelude> :k Data.Array.IArray.IArray
>>> Data.Array.IArray.IArray :: (* -> * -> *) -> * -> Constraint
>> This last one I do not follow.  A short explanation and/or a pointer
>> to some documentation would help.
> 'IArray' is a typeclass, in this case essentially used as means of providing
> a standard interface to arrays.  The kind signature above means that the
> interface is parametrised by the array constructor (of kind * -> * -> *,
> like UArray, see above) and the type of the index.  If you give 'IArray' an
> array constructor and an index type you get a 'Constraint', in this case a
> typeclass context which essentially provides you with an interface to access
> your 'UArray' with.
> However, these constructions are not suitible for making an instance of
> Foldable, because Foldable would require implementing functions that are
> uniform in the way they treat the type argument 'e', and as we've seen, the
> usage of 'e' in the datatype and interface is far from uniform.
> Hope that helps,
> Tom

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