Can Template Haskell and RULES be combined?

It would be nice if there was something like this:

{-# RULES
    "(literal*)" forall n.
        (n*) = $( makeOptimizedMultiplication [|| n ||] ) #-}

where makeOptimizedMultiplication is a Template Haskell function that, if n is an integer literal, produces an optimized multiplication function for that literal. However, that code above gives a VarE corresponding to the n in the quoter rather than the AST of the n that's actually being matched by the rewrite rule, so I can't get a LitE that way.

This sort of thing would come in very handy for a large fixed-width integer type, because I don't think GHC and LLVM will know that standard fixed-width integer optimizations apply to it. For example, with a Word65536 type, multiplying blindly at runtime by the literal 1099511627777 requires a lot of extra work. At the very least, all 65536 bits of that literal have to be read and dealt with pretty blindly.

However, at compile time, the compiler can take the time to figure out that (1099511627777*) can be replaced with the much, much lighter (\x -> shiftL x 40 + x) (and not only that, but if Word65536s are stored in a byte array, 1 byte evenly divides the shift amount of 40 bits, which can be exploited to speed things up greatly). With appropriate RULES and Template Haskell functions, the compiler gains the ability to apply lots of fixed-width integer optimizations.

Since the above sort of code doesn't work, is there some other way to do this that I don't know about? If not, can the feature be added?

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The only downside I can think of is that the user of the Word65536 type might not knowingly be expecting Template Haskell to be at work since they didn't use any splices or quoters and might not have even included {-# LANGUAGE TemplateHaskell #-} in their code, so perhaps some easy method of consenting to the application of TH-based rewrites might be in order.

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Trac field	Value
Version	8.4.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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