If I have this Foo.hs:
------------------------------- module Foo (foo) where
import Word (Word8) import Control.Monad.ST (ST) import Data.Array.ST (STUArray, writeArray)
foo :: STUArray s Int Word8 -> [Word8] -> Int -> ST s () foo arr ps i = writeArray arr i w where i' = 4 * i w = ps !! i' -------------------------------
and I compile with (GHC 6.2 and a reasonably recent CVS)
ghc -O -funbox-strict-fields -Wall -c Foo.hs -ddump-simpl
then part of the output is:
let { n :: GHC.Prim.Int# Str: DmdType n = GHC.Prim.*# 4 ww3 } in case GHC.Prim.<# n 0 of wild1 {
Is this really a lazy let, or is there some magic going on that means it is actually done strictly as it's an Int#?
Yes, this let will be done strictly. Indeed, any let which is sure to be demanded (according to the Demand value of the Id) will turn into a case in STG, and similarly for an expression in the argument position of a strict function call. I believe the motivation was that this makes things much easier for the simplifier, but Simon PJ will be able to elaborate I'm sure.
Like how ISTR Int#s always appear to have strictness L (these inconsitencies make things much more difficult as a user IMO, incidentally).
ISTR? Cheers, Simon