GitLab

I have

{-# LANGUAGE MagicHash #-}

module Main where

import GHC.Exts

five = I# (2# +# 3#)

{-# RULES "funky" (+#) = (*#) #-}

main :: IO ()
main = print five

When I ghc -O Scratch.hs -main-is Scratch -fforce-recomp -ddump-rule-rewrites, I get

Loaded package environment from /Users/rae/.ghc/x86_64-darwin-8.10.1/environments/default
[1 of 1] Compiling Scratch          ( Scratch.hs, Scratch.o, Scratch.dyn_o )
Rule fired
    Rule: funky
    Module: (Scratch)
    Before: GHC.Prim.+# ValArg 2# ValArg 3#
    After:  GHC.Prim.*#
    Cont:   Stop[BoringCtxt] GHC.Prim.Int#
Rule fired
    Rule: *#
    Module: (BUILTIN)
    Before: GHC.Prim.*# ValArg 2# ValArg 3#
    After:  6#
    Cont:   Stop[BoringCtxt] GHC.Prim.Int#
Rule fired
    Rule: Class op show
    Module: (BUILTIN)
    Before: GHC.Show.show TyArg GHC.Types.Int ValArg GHC.Show.$fShowInt
    After:  GHC.Show.$fShowInt_$cshow
    Cont:   ApplyToVal nodup x_aN7
            Stop[BoringCtxt] GHC.Base.String
Linking Scratch ...
ld: warning: directory not found for option '-L/opt/local/lib/'
ld: warning: text-based stub file /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libQuadrature.tbd and library file /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libQuadrature.dylib are out of sync. Falling back to library file for linking.
rae:14:29:21 ~/temp> 

Note the first rule rewrite. It says

    Before: GHC.Prim.+# ValArg 2# ValArg 3#
    After:  GHC.Prim.*#

Eek! At first, I thought we were bound for a core-lint error. Actually, all is well: it's just a printing problem. But I was thrown off the scent of some other misbehavior by this one.