repa: fromVector

[Christian Höner zu Siederdissen]

Hi,

there is a ticket for 'vector' available with what I need:
http://trac.haskell.org/vector/ticket/15

basically, I need a generalised scan that makes available a partially
initialized vector for each new element.

gen_scan :: Vector v a => Int -> (v a -> a) -> v a

Only that 'a' tends to be a 5-tuple ;-)

A function along those lines is extremely useful in a number of cases. I
hope to have some time to actually dig through the repa sources and see
what is possible.

Gruss,
Christian

* Ben Lippmeier <benl at ouroborus.net> [20.05.2011 04:14]:
> 
> On 19/05/2011, at 8:27 PM, Christian Höner zu Siederdissen wrote:
> 
> > I'd like to use repa in a rather perverted mode, I guess:
> > 
> > for my programs I need to be able to update arrays in place and
> > repeatedly perform operations on them.
> > Right now, it basically works like this (in ST):
> > 
> > - create unboxed space using "primitive" (same as unboxed vectors)
> > - unsafefreeze unboxed space
> > - perform calculations on frozen, immutable space
> > - write result into mutable space (which is shared with the unsafefrozen
> >  space)
> 
> If you care deeply about inplace update, then you could use the parallel array filling functions directly. The ones in  D.A.Repa.Internals.Eval*.hs. For 2D images, use the fillVectorBlockwiseP [1] or fillCursoredBlock2P.
> 
> 
> fillVectorBlockwiseP 
> 	:: Elt a
> 	=> IOVector a		-- ^ vector to write elements into
> 	-> (Int -> a)		-- ^ fn to evaluate an element at the given index
> 	-> Int			-- ^ width of image.
> 	-> IO ()
> 
> 
> -- | Fill a block in a 2D image, in parallel.
> --   Coordinates given are of the filled edges of the block.
> --   We divide the block into columns, and give one column to each thread.
> fillCursoredBlock2P
> 	:: Elt a
> 	=> IOVector a		-- ^ vector to write elements into
> 	-> (DIM2   -> cursor)		-- ^ make a cursor to a particular element
> 	-> (DIM2   -> cursor -> cursor)	-- ^ shift the cursor by an offset
> 	-> (cursor -> a)		-- ^ fn to evaluate an element at the given index.
> 	-> Int			-- ^ width of whole image
> 	-> Int			-- ^ x0 lower left corner of block to fill
> 	-> Int			-- ^ y0 (low x and y value)
> 	-> Int			-- ^ x1 upper right corner of block to fill
> 	-> Int			-- ^ y1 (high x and y value, index of last elem to fill)
> 	-> IO ()
> 
> 
> Actually, it might be worthwhile exporting these in the API anyway.
> 
> [1] http://code.ouroborus.net/repa/repa-head/repa/Data/Array/Repa/Internals/EvalBlockwise.hs
> [2] http://code.ouroborus.net/repa/repa-head/repa/Data/Array/Repa/Internals/EvalCursored.hs
> 
> 
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