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

Dan Doel dan.doel at
Mon Feb 15 13:40:56 UTC 2016

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.

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

On Mon, Feb 15, 2016 at 5:12 AM, Simon Marlow <marlowsd at> wrote:
> On 13 February 2016 at 08:32, Christopher Allen <cma at> wrote:
>> 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:
> Cheers
> Simon
>> This is on by default and insofar as I've been able to try, it's avoidable
>> 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 parameters
>> 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 think
>> 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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