Hi (All these results are from GHC 6.9.20071226, but suspect they hold for 6.9.* and 6.8) The following code: test = head "neil" Produces (with -O2): Text.HTML.TagSoup.Development.Sample.test = case GHC.Base.unpackCString# "neil" of wild_aaU { [] -> GHC.List.badHead @ GHC.Base.Char; : x_aaY ds1_aaZ -> x_aaY } However: test = head ['n','e','i','l'] Produces: Text.HTML.TagSoup.Development.Sample.test = GHC.Base.C# 'n' The packing of the string has damaged the optimisation. Is there anyway to mitigate this? I ideally want to do this in RULES to derive an efficient parser from a set of parser combinators, and the unpackCString# really gets in the way of future optimisations. I could imagine adding two rules to the simplifier: case unpackCString# "" of ==> case [] of case unpackCString# "xyz" of ==> case (C# 'x': unpackCString# "yz") of Then the simplifier could recover the optimised version. Thanks Neil