Dear Cafe, * how to un-uglify the following program? * and is it even ugly enough? The following is some artificial example but I think it illustrates some general questions - for which I don't have answers. Do you? I wanted a really fast implementation of a vector addition system where the "vector" contains small numbers only. and I want it packed into a machine word (and finally put these into Data.IntSet). I can write this in low-level Haskell (I did), and I could even use some low-level language (I also did) but I was hoping that I don't need to. https://gitlab.imn.htwk-leipzig.de/waldmann/tf/-/blob/master/TF4.hs#L108 * this is too low-level: This is for vectors of one fixed size - but I want the code to be more generic, and have the compiler do the specialization/unrolling. * and not low-level enough: How do I tell GHC to pack (coerce?) `data Pos` into `Word64`? (It's not https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/pragmas.html#unpack-... ?) And would it help? Or is it even needed? If I have the data spread over several words, it could still be fine - as long as it's kept in registers? How can I then check that "it's fine"? Yes I know, just look at the core. (That seems to be the ultima ratio of https://ghc.gitlab.haskell.org/ghc/doc/users_guide/hints.html#faster-produci...) But is there/could there be some higher-level mechanism? Perhaps a (type?) annotation that guarantees or checks that core (locally) looks "good"? (does not allocate, ...) NB: For the concrete application (evaluation of positions in some game): 1. graph size is exponential, so a linear speed-up won't really help. 2. brute-force gameplay should be replaced by some symbolic evaluation and simplification. That is in fact my goal. But - as long as we don't have this, the program may be useful for generating conjectures. - J.