[Haskell-cafe] fast Array operations: foldl, drop

[Jules Bean]

Henning Thielemann wrote:
> I thought operations like "foldl'" and "drop" must be very fast on arrays
> (especially UArray) with appropriate pointer tricks, I mean pointer
> incrementing instead of indexing for "foldl'" and a pointer into the array
> for "drop". Is it planned to add such functions? Ok, if "foldl f x .
> elems" and "listArray (i,sufficientlyBig) . drop n . elems" are fused to
> high speed code, then these functions do not need to materialize in the
> API.
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As far as I'm aware, our arrays don't support any kind of zero-copy 
slicing, which is what your 'drop' trick amounts to.

Zero-copy slicing would seem like an obviously nice thing to have for 
IArrays, though, I agree. For various different kinds of slices 
including projections.

ByteString supports zero-copy substring, but it's based on ForeignPtr 
not UArray.

On the fusing point I believe that the stream fusers believe they can 
make things like foldl . elems fuse but I'm not sure.

Jules