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Today's Topics:
1. Re: explaining effects (Alexander Berntsen)
2. Re: explaining effects (emacstheviking)
3. Re: explaining effects (mike h)
4. Re: Extend instance for List (Kim-Ee Yeoh)
----------------------------------------------------------------------
Message: 1
Date: Tue, 15 Dec 2015 09:55:44 +0100
From: Alexander Berntsen <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] explaining effects
Message-ID: <[email protected]>
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On 12/12/15 14:38, Daniel Bergey wrote:
> Why do you prefer "effect" to "side effect"?
FWIW, I say "effect" rather than "side effect" when talking about
Haskell, because in Haskell effects happen when you want them, not as
an unforeseen side-effect as a result of the complexity inherent to
the source code.
It is often said that having an effect is "difficult" in Haskell. But
really, it's just that if you are launching missiles in Haskell, *you
actually mean to*. It didn't happen because you wanted to increment i
and then "oops, stuff happened".
- --
Alexander
[email protected]
https://secure.plaimi.net/~alexander
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Message: 2
Date: Tue, 15 Dec 2015 09:15:19 +0000
From: emacstheviking <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] explaining effects
Message-ID:
<caeieuujk6gkush0_b2-bonb6q9zazzkxt1tfn_c9uo+ywkb...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
good answer! Having worked on a lot of embedded microprocessor systems
over the years... that's exactly the kind of thing you don't want and
sometimes all too easy to do by mistake when writing C or assembler!
Good answer!
:)
On 15 December 2015 at 08:55, Alexander Berntsen <[email protected]>
wrote:
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> Hash: SHA512
>
> On 12/12/15 14:38, Daniel Bergey wrote:
> > Why do you prefer "effect" to "side effect"?
> FWIW, I say "effect" rather than "side effect" when talking about
> Haskell, because in Haskell effects happen when you want them, not as
> an unforeseen side-effect as a result of the complexity inherent to
> the source code.
>
> It is often said that having an effect is "difficult" in Haskell. But
> really, it's just that if you are launching missiles in Haskell, *you
> actually mean to*. It didn't happen because you wanted to increment i
> and then "oops, stuff happened".
>
> - --
> Alexander
> [email protected]
> https://secure.plaimi.net/~alexander
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Message: 3
Date: Tue, 15 Dec 2015 09:44:14 +0000 (UTC)
From: mike h <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] explaining effects
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="utf-8"
A good and witty answer :)
On Tuesday, 15 December 2015, 9:16, emacstheviking <[email protected]>
wrote:
good answer! ? Having worked on a lot of embedded microprocessor systems over
the years... that's exactly the kind of thing you don't want and sometimes all
too easy to do by mistake when writing C or assembler!
Good answer!
:)
On 15 December 2015 at 08:55, Alexander Berntsen <[email protected]> wrote:
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Hash: SHA512
On 12/12/15 14:38, Daniel Bergey wrote:
> Why do you prefer "effect" to "side effect"?
FWIW, I say "effect" rather than "side effect" when talking about
Haskell, because in Haskell effects happen when you want them, not as
an unforeseen side-effect as a result of the complexity inherent to
the source code.
It is often said that having an effect is "difficult" in Haskell. But
really, it's just that if you are launching missiles in Haskell, *you
actually mean to*. It didn't happen because you wanted to increment i
and then "oops, stuff happened".
- --
Alexander
[email protected]
https://secure.plaimi.net/~alexander
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Message: 4
Date: Tue, 15 Dec 2015 17:36:58 +0700
From: Kim-Ee Yeoh <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Extend instance for List
Message-ID:
<CAPY+ZdTjvwZNt7=5RcAjyiLtmsgu8auoayWp9d=70yfaoav...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Tue, Dec 15, 2015 at 12:43 AM, Graham Gill <[email protected]>
wrote:
I guessed that the correct Extend instance for List is needed for Comonad,
> but didn't have any intuition about it.
>
List is not comonadic. In this case, the function copure must be of type
[a] -> a, which must necessarily be partial.
(Non-empty lists, on the other hand, are comonadic.)
Graham's "Extend" -- I'll explain the scare quotes in a minute -- instance
for List obeys the associative law. So it's a valid instance but a bit
boring. The exercise asks for an interesting instance.
The way the NICTA course is structured, there's no mention of the
> dependence between "extend" and "copure" (equivalent to extract and
> duplicate I suppose) via the Comonad laws when considering Extend first by
> itself.
>
It's a bit terse, but you can find "class Extend f => Comonad f" in
Comonad.hs. After all, we're only looking at the exercises. The live
lecture version probably does talk about the dependence.
> I'm not knocking the NICTA course. I've found it useful. A quick paragraph
> or two as you've written, stuck into the source files as comments, would
> improve it.
>
Most folks are neutral about the course. If parts of it work for you,
great. If not, no worries. The whole comonadic business is a bit obscure
and some of the strongest haskell programmers don't bat an eyelid over not
knowing it.
p.s. "Extend" doesn't agree with the CT literature. See the paragraph that
starts "The dual problem is the problem of lifting a morphism" here:
http://ncatlab.org/nlab/show/extension
But calling it a "lift" or "lifting" will only add to the confusion since
monad transformers got first dibs on the terminology. Which is why you
sometimes see "coextend" or (for the flipped version) "cobind".
-- Kim-Ee
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