Re: New type of expressions containing (error ...) includes noisy implicit parameter

On 15 February 2016 at 13:40, Dan Doel wrote:

But that is a consequence of call stacks being implemented by threading a parameter through the calls, not of the fact that it shows up in the type. Without the change in type, the programmer wouldn't be informed there is any difference without looking at the core.

I'm not suggesting that the parameter should be hidden in the type! I agree that would be much worse. No, I'm arguing that it was bad that we lost sharing just by using undefined (or error). If adding a call to error somewhere in your code changes its asymptotic performance, that's at the very least surprising. Apologies for diverting the discussion. I also think it's potentially bad from an educational point of view that error/undefined have these non-standard types, in fact I argued against using implicit call stacks in other functions in base for this reason (amongst others), and fortunately we've limited it to just these. Cheers Simon In other words, this gives one reason why users _do_ care that stack

information is being carried around, and might want to see it in the type; it can have a runtime performance impact (and likely not just for CAFs).

-- Dan

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On 13 February 2016 at 08:32, Christopher Allen wrote:

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Prelude> let myList = [1, 2, 3 :: Integer] Prelude> let myList' = myList ++ undefined Prelude> :t myList myList :: [Integer] Prelude> :t myList' myList' :: (?callStack::GHC.Stack.Types.CallStack) => [Integer]

Yes, and I think perhaps an even more worrying problem here is that by adding the reference to undefined, myList went from being a thunk to being a function. That is, it will be re-evaluated each time it it used. I made a ticket about this: https://ghc.haskell.org/trac/ghc/ticket/11383

Cheers Simon

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This is on by default and insofar as I've been able to try, it's

avoidable

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in a default GHCi 8.0 REPL session. I'm glad I caught this before our book goes to print in a couple months. We'd managed to avoid talking about implicit parameters in 1,100+ pages of book but now we're forced to acknowledge their existence in the 4th of 32 chapters.

This slipped past the radar more stealthily than the earlier stages of BBP did for 7.10. I was hearing about BBP on the GHC Trac pretty early on for months on end. Was the thinking that people still used implicit

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for anything or taught them? On the one hand, this is a nice change and something I personally attempted (and failed) to make easier in GHC 7.10. The implementation making the types noisy rankles and didn't seem necessary when I investigated it between 7.8 and 7.10.

Could you warn us when (educationally relevant?) stuff like this is coming down the pipe before the RC please? Ideally during the design phase. I

On Mon, Feb 15, 2016 at 5:12 AM, Simon Marlow wrote: parameters think

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this was discussed as part of FTP to avoid future debacles.

This isn't just a pedagogical problem, this is a UX problem. The users don't _care_ that call stack information is being carried around. Why would they? It happens without any mention in the types in almost every other programming language.

--- Chris Allen

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