When profiling this code: consolidateRPR :: [Sample] -> [Sample] smplUnionRecursRev :: [Sample] -> Sample -> [Sample] sortSamps :: [Sample] -> [Sample] smplSetUnion :: [Sample] -> [Sample] smplSetUnion = consolidateRPR . (foldl smplUnionRecursRev []) . sortSamps I'm getting this: smplSetUnion Data.RandProc 1038 32794 0.7 0.2 76.9 72.6 consolidateRPR Data.RandProc 1041 32794 0.0 0.0 8.2 11.8 smplUnionRecursRev Data.RandProc 1040 918112 8.8 5.6 8.8 5.6 sortSamps Data.RandProc 1039 1081514 21.1 55.0 59.2 55.0 I'm confused as to why `sortSamps` is being called so many times. I would expect it to be called 32794 times, just like `consolidateRPR`. It's as if having a `foldl` to the left of `sortSamps` in the composition chain is causing it to execute more than once, per call of `smplSetUnion`, but I wouldn't expect that. This doesn't change anything: smplSetUnion = consolidateRPR . (\ss -> foldl smplUnionRecursRev [] ss) . sortSamps (I thought it might explicitly close the `foldl`.) In fact, even this: smplSetUnion ss = consolidateRPR $ foldl smplUnionRecursRev [] ss' where ss' = sortSamps ss doesn't change anything. `sortSamps` still gets entered 1081514 times! Can anyone help clear up this newbie's fog? Thanks, in advance! -db