Need rewrite rules for fromIntegral

[j6carey]

It seems that newtypes such as Seconds need rewrite rules in order for fromIntegral to work efficiently. Perhaps something inspired by these general rules for Int64 from GHC.Int:

{-# RULES
"fromIntegral/a->Int64" fromIntegral = \x -> case fromIntegral x of I# x# -> I64# x#
"fromIntegral/Int64->a" fromIntegral = \(I64# x#) -> fromIntegral (I# x#)
  #-}

Without such rules, we temporarily create an indefinite-precision Integer:

module T where

import Data.Hourglass
import Data.Word

foo :: Word32 -> Seconds
foo = fromIntegral

$ ghc -O2 -ddump-simpl -dsuppress-all t.hs
[1 of 1] Compiling T                ( t.hs, t.o )

==================== Tidy Core ====================
Result size of Tidy Core = {terms: 12, types: 6, coercions: 2}

foo
foo =
  \ ds_a1Ed ->
    case ds_a1Ed of _ { W32# x#_a1Eg ->
    case integerToInt (wordToInteger x#_a1Eg)
    of wild1_a1TV { __DEFAULT ->
    (I64# wild1_a1TV) `cast` ...
    }
    }

(Tested using hourglass-0.2.9.)

[vincenthz]

Good idea. Any chance you can make that in a PR ?