The following discussion from !12685 Pmc: Improve implementation of -Wincomplete-record-selectors ( #24824, #24891) - [ ] @simonpj started a [discussion](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/12685#note_583762 "Pmc: Improve implementation of -Wincomplete-record-selectors (#24824, #24891)"): (+1 comment) > Wow, this is brutal -- calling `decomposeRecSelHead` on every single application. > > Moreover, something rather similar happens _immediately_ afterwards, in `mkCoreAppsDs`. > > I wonder if we could do something like: > > ``` > dsExpr e@(HsApp {}) = dsApp e [] > > dsApp (HsApp fun arg) core_args > = do { arg' <- dsLExpr arg > ; dsApp fun (arg':core_args) } > > dsApp (HsVar fun) core_args > | fun == seqId = ... > | fun == noInlinId = ... > | isRecSel fun = ... > | fun = hasFieldId = ... > ``` > > We might need to duplicate a bit of `dsHsWrapper`, namely the cases for dictionary and type application, but it might be worth it. Basically, just accumulate the arguments, and dispatch on the function at the head. > > There is yet more stuff in `dsHsWrapped` which I can't figure out. But it calls `warnAboutIdentities` that would also fit nicely in the `HsVar` case of `dsApp` above. We seem to be doing something very similar (look at the function, do something based on that) in at least three different places. > > I suppose this is an unforced refactoring but the current story grates on me a bit. It seems bad to do the same thing in 3 different ways! > > **Moreover** the fancy switching on and off of flags to deal with the 1/2/3 ovderlap might not be needed any more: we just have three equations in the Var case of dsApp. > > The more I think about this the more I like it! and also: > it has nothing to do with perf. It's the intellectual overhead of doing the same thing in three different ways!