[GHC] #15547: A function `nat2Word# :: KnownNat n => Proxy# n -> Word#`

[GHC]

#15547: A function `nat2Word# :: KnownNat n => Proxy# n -> Word#`
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           Reporter:  ChaiTRex       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 It would be nice if there were the function (perhaps in `GHC.Prim`)
 `nat2Word# :: KnownNat n => Proxy# n -> Word#` that was essentially `(\ W#
 w# -> w#) . fromInteger . natVal`, except that it produces the `Word#` at
 compile-time rather than runtime and that it works with `Proxy#` (for
 [https://hackage.haskell.org/package/base-4.11.1.0/docs/GHC-
 Exts.html#t:Proxy-35- its no-runtime-representation, totally-free nature])
 instead of `Proxy`.

 This would enable compile-time computations on `Nat`s to produce a literal
 `Word#` without any runtime expense at all, which seems nice if you're
 dealing with primitives ''because'' you want to avoid runtime expense.

 == Motivating example

 I'm learning primitive types and type-level arithmetic by making a simple
 set of fixed-width integer types where the type contains a `Nat` for the
 number of bits in the fixed-width integer.

 Going from the following `Nat`s to their corresponding `Word#`s at compile
 time even when optimizations are turned off during development would be
 very nice:

 ||=             `Div (n + 63) 64` \\=||the highest index we should try to
 access via `indexWordArray#`                        ||
 ||=          `Mod (n - 1) 64 + 1` \\=||except when `n == 0`, the number of
 bits actually used in the \\ most-significant word ||
 ||= `2^(Mod (n + 63) 64 + 1) - 1` \\=||the unsigned integer narrowing
 bitmask to use on the          \\ most-significant word ||

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15547>
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