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-0.5.0.0/docs/src/Data-Array-Base.ht...
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