[Haskell-cafe] New type of expressions containing (error ...) includes noisy implicit parameter

[Christopher Allen]

>subtle difference means that you do need to know about this due to how
stacks work in GHC. If you use error and then don't also add that
constraint yourself to your function, youll only see part of the stack.

It is unfortunate that the design forces this implementation detail on
users trying to apply a plain old function.


I updated my GHC 8.0.1 candidate, got this:

$ ghci
GHCi, version 8.0.0.20160204: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/callen/.ghci
Prelude> let myList = [1..5 :: Integer]
Prelude> let myList' = myList ++ undefined
Prelude> :t myList'
myList' :: (?callStack::GHC.Stack.Types.CallStack) => [Integer]

> changed to use constraint kinds and type alias the implicit parameter
behind a nicer API.

Even if this has happened, we're still talking about the fourth chapter
which is prior to types, typeclasses, constraints - everything. None of
this means anything to a beginner being shown how to use undefined. The
book is trying to make sure _anyone_ can learn Haskell, it's not an
advanced DSLs book that introduces DataKinds.

The implementation is leaking all the way to the fourth chapter of a book
for complete beginners. Users will not be sympathetic when the
implementation elides information because the user failed to speak to the
nasal demons in the correct accent.



On Sat, Feb 13, 2016 at 3:04 AM, Oliver Charles <ollie at ocharles.org.uk>
wrote:

> Just a clarification - the presence of that parameter does not say "I use
> call stack" but rather "I need *a* call stack". This subtle difference
> means that you do need to know about this due to how stacks work in GHC. If
> you use error and then don't also add that constraint yourself to your
> function, youll only see part of the stack. Hence it more than just UX
> noise - I have a decision to make, and building a call stack gas a cost.
>
> Also, is this RC2? I thought this was now changed to use constraint kinds
> and type alias the implicit parameter behind a nicer API.
>
> On Sat, 13 Feb 2016 8:51 am Christopher Allen <cma at bitemyapp.com> 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]
>>
>> 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.
>>
>> We checked after the previous thread where ($) occurred in the book. ($)
>> is in the second chapter of the book, which is the first chapter of Haskell
>> code. Now we have to hand-wave something completely impossible for them to
>> understand (chapter 2 is expressions, types are chapter 5) or edit ($) out
>> of the book until they can understand it. We can't write it completely out
>> of the book because ($) gets used all over the place and we don't want
>> encountering it to throw them off.
>>
>>
>> --- Chris Allen
>>
>> _______________________________________________
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>> Haskell-Cafe at haskell.org
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>>
>


-- 
Chris Allen
Currently working on http://haskellbook.com
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