> On 19 Sep 2018, at 1:07 am, Daniel Cartwright <[email protected]> wrote: > > in GHC.Prim, there is a primitive 'touch#'. It seems that it is used to keep > pointers alive, and takes something of any runtime representation, and a > State# RealWorld, but there's really no documentation on it, so my idea of > what it does is just a guess. Can anyone explain what exactly it does?
It’s used to place a dependency on a computed value, to prevent the GHC simplifier from moving the binding that computes it around. This is sometimes helpful when writing array fusion systems. I mentioned it briefly in http://benl.ouroborus.net/papers/2011-stencil/stencil-haskell2011.pdf <http://benl.ouroborus.net/papers/2011-stencil/stencil-haskell2011.pdf> Cheers, Ben.
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