Carter Schonwald | 21 Aug 20:14 2012
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question about type lits and optimization in ghc

Hey GHC folks!

 
In the case where type lits (naturals specifically) are statically known (rather than abstract), is it possible to have a sort of static copy propagation / Singelton simplification happen at compile time? (Ie can GHC do such things with  type lits as presently implemented?)

a simple example might be that a statically sized matrix, and being able to specialize the stride and bounds checking at compile time (pseudo code example follows)

data Mat (row::Nat) (col::Nat) elem = ... some vector representation


index:: (Mat r c  el) -> (Word,Word) -> el
index mt ix = ... some stride based indexing calculation that gets at the "r"  and "c" numbers via the singletons  to translate the ix tuple into the index into the underlying vector
representation
{-# perhaps an inline pragma? #-}

indexSpec3x3:: (Mat 3 3 el) -> (Word, Word) -> el
indexSpec3x3 smat ix = smat `index` ix

under what circumstances can i get something like the body of indexSpec3x3 to use the static type level natural number machinery to partially specialize the computation?
Does my question even make sense as phrased?

thanks!

-Carter



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