[Haskell-cafe] What is your favourite Haskell "aha" moment?

[Conal Elliott]

Concurrency *can* be defined in Haskell proper (the denotative subset), but
not compatibly with that "source code" and probably not at all with the
current particulars of IO. With the source code in question, even the "real
world" cannot evolve concurrently with program execution, let alone
different threads with an IO computation.

On Wed, Jul 11, 2018 at 7:03 PM, Brandon Allbery <allbery.b at gmail.com>
wrote:

> In the presence of concurrency, IO requires runtime support and can't be
> simply defined in Haskell proper.
>
> On Wed, Jul 11, 2018 at 9:59 PM Vanessa McHale <vanessa.mchale at iohk.io>
> wrote:
>
>> I'm not sure I follow. Do you mean that IO is not a monad because
>> equivalence of values cannot be defined? Or is it something deeper?
>> On 07/11/2018 05:19 PM, Conal Elliott wrote:
>>
>> > The fact that you can define the IO monad in Haskell was quite a
>> revelation.
>>
>> But it's *not* a fact. It's a lie. And one of the most devious sort,
>> since the source code appears to agree. The purported definition couldn't
>> possibly explain concurrency.
>>
>> On Wed, Jul 11, 2018 at 7:21 AM, Vanessa McHale <vanessa.mchale at iohk.io>
>> wrote:
>>
>>> I find it quite elegant! The fact that you can define the IO monad in
>>> Haskell was quite a revelation. And it's especially nice when paired with a
>>> demonstration of C FFI (where you might *need* to sequence side effects
>>> such as freeing a value after it has been read).
>>>
>>> newtype IO <http://hackage.haskell.org/package/ghc-prim-0.5.2.0/docs/src/GHC.Types.html#IO> a <http://hackage.haskell.org/package/ghc-prim-0.5.2.0/docs/src/GHC.Types.html#local-6989586621679009802> = IO <http://hackage.haskell.org/package/ghc-prim-0.5.2.0/docs/src/GHC.Types.html#IO> (State# <http://hackage.haskell.org/package/ghc-prim-0.5.2.0/docs/src/GHC.Prim.html#State%23> RealWorld <http://hackage.haskell.org/package/ghc-prim-0.5.2.0/docs/src/GHC.Prim.html#RealWorld> -> (# State# <http://hackage.haskell.org/package/ghc-prim-0.5.2.0/docs/src/GHC.Prim.html#State%23> RealWorld <http://hackage.haskell.org/package/ghc-prim-0.5.2.0/docs/src/GHC.Prim.html#RealWorld>, a <http://hackage.haskell.org/package/ghc-prim-0.5.2.0/docs/src/GHC.Types.html#local-6989586621679009802> #))
>>>
>>>
>>> On 07/11/2018 09:14 AM, Stefan Monnier wrote:
>>>
>>> In a few weeks I'm giving a talk to a bunch of genomics folk at the Sanger
>>> Institute<https://www.sanger.ac.uk/> <https://www.sanger.ac.uk/> about Haskell.   They do lots of
>>> programming, but they aren't computer scientists.
>>> I can tell them plenty about Haskell, but I'm ill-equipped to answer the
>>> main question in their minds: why should I even care about Haskell?  I'm too
>>> much of a biased witness.
>>>
>>> I don't much like the monad solution for side-effects, but if those guys
>>> might have some knowledge of the horror of concurrent programming with
>>> locks, the STM system would be a good candidate.
>>>
>>>
>>>         Stefan
>>>
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>>>
>>> --
>>>
>>>
>>>
>>> *Vanessa McHale*
>>> Functional Compiler Engineer | Chicago, IL
>>>
>>> Website: www.iohk.io <http://iohk.io>
>>> Twitter: @vamchale
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>>>
>>> [image: Input Output] <http://iohk.io>
>>>
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>>
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>
>
>
> --
> brandon s allbery kf8nh                               sine nomine
> associates
> allbery.b at gmail.com
> ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad
> http://sinenomine.net
>
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