Proposal: Add IsList instance for ZipList

Andreas Abel abela at
Fri Jun 12 07:06:01 UTC 2015

I'd like to avoid situations where adding a LANGUAGE pragma changes the 
semantics of code  Is this one?

On 12.06.2015 03:49, Henning Thielemann wrote:
> On Thu, 11 Jun 2015, Edward Kmett wrote:
>> I personally strongly believe in keeping the Applicative and Monad for
>> a type compatible. Otherwise why the heck did we make Applicative a
>> superclass of Monad?
> I was extending the thought of treating a list like it would have
> ZipList semantics. I thought loudly about what I would have done if I
> could re-invent Haskell 98. If list would have an Applicative instance
> with ZipList semantics then it would not have a Monad instance, at all.
>> In this case the thing David wants only makes any difference if you
>> explicitly turn on OverloadedLists, so if you never use that extension
>> you'll never care about the instance anyways.
> When I speak about surprises then I mean the situation where I read code
> of others, since I am not as much surprised about my own code. In this
> case someone else has enabled OverloadedLists.
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Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel at

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