#14941: Switching direct type family application to EqPred (~) prevents inlining in code using vector (10x slowdown) -------------------------------------+------------------------------------- Reporter: nh2 | Owner: davide Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nh2): Replying to [comment:9 simonpj]:
How important is it? I'm tempted to say "don't write types like that"!
From the user's perspective, I can put forward two use cases: 1. Type level let bindings `(..., VG.Mutable v ~ vm) => ... -> vm (PrimState m) e -> ...` From the issue description. This is essentially type-signature `let`-binding, allowing me to write the type signature more clearly/legibly, especially if `vm` is used multiple times. 2. Refactorings For the code in comment 4, `(val ~ Int) =>`, this arose from a refactoring where I replaced `Int` by `val` across a large code base, and then in the top-level-ish function set `val ~ Int` to check if the performance was unimpacted. I'd probably be OK without this being fixed, now that I know what's going on, but if it's not fixed, we somehow have to do a much better job at giving warnings or educating people that you can't just apply a substitution principle when "cleaning up" type signaturees with `~`. I incorrectly assumed this would be type-level only, and as a result had to start a multi-day investigation of where my performance went because I expected any mistake on my side but not this. Especially when writing code using `vector` whose type family heavy API, in my opinion, almost begs for `~` to be used to achieve readable signatures. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14941#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler