[Haskell-cafe] do-notation for building monoid values
Oliver Charles
ollie at ocharles.org.uk
Fri Mar 9 09:24:30 UTC 2018
Another mconcat user here. The only place it does fall down a bit is when
you want something to be optional. In a monad we have `when`, but I don't
know of any similar combinator for monoids. Of course, it's just `if x then
y else mempty`, but I always end up defining a when-like function over and
over again.
On Fri, Mar 9, 2018 at 12:53 AM Brandon Allbery <allbery.b at gmail.com> wrote:
> The mconcat method is also how xmonad handles its ManageHooks, which are
> basically Endo Monoids.
>
> On Thu, Mar 8, 2018 at 8:48 AM, Andrew Martin <andrew.thaddeus at gmail.com>
> wrote:
>
>> I do the same thing that Merijn does. It works really well.
>>
>> On Thu, Mar 8, 2018 at 8:08 AM, Merijn Verstraaten <
>> merijn at inconsistent.nl> wrote:
>>
>>> I always just use mconcat and lists, so instead of:
>>> > complicatedValue = execWriter $ do
>>> > component1
>>> > component2
>>> > modifier $ do
>>> > component3
>>> > component4
>>>
>>> I write:
>>>
>>> complicatedValue = mconcat
>>> [ component1
>>> , component2
>>> , modifier . mconcat $ [component3, component4]
>>> ]
>>>
>>> Alternatively, if component3 and component4 are really long or lots of
>>> them linewrap those too:
>>>
>>> complicatedValue = mconcat
>>> [ component1
>>> , component2
>>> , modifier . mconcat $
>>> [ component3
>>> , component4
>>> ]
>>> ]
>>>
>>> Cheers,
>>> Merijn
>>>
>>> > On 8 Mar 2018, at 14:02, Olaf Klinke <olf at aatal-apotheke.de> wrote:
>>> >
>>> > Dear cafe,
>>> >
>>> > prompted by a discussion with the author of blaze-markup [1] I
>>> realized a pattern and would like to know whether other haskellers have
>>> exploited this/find this useful:
>>> >
>>> > For every monoid m, the types Writer m () and m are isomorphic as
>>> types via tell and execWriter. Moreover, Writer m is a monad if and only if
>>> m is a monoid. For every monad t, the type t () is a monoid with
>>> > mempty = return ()
>>> > mappend = (>>).
>>> > In the particular case of Writer m () and m, the isomorphism of
>>> Haskell types is in fact an isomorphism of monoids, that is, the functions
>>> tell and execWriter preserve the monoid operations.
>>> >
>>> > This enables us to use do-notation in building complicated values of
>>> any monoid type, e.g. Text or any other syntax. Instead of writing
>>> >
>>> > complicatedValue = component1 <>
>>> > component2 <>
>>> > modifier (component3 <> component4)
>>> >
>>> > one can write
>>> >
>>> > complicatedValue = execWriter $ do
>>> > component1
>>> > component2
>>> > modifier $ do
>>> > component3
>>> > component4
>>> >
>>> > Should such an idiom be encouraged/discouraged? How do you handle the
>>> construction of monoid values (especially text-like) with interspersed
>>> function applications (e.g. show, prettyprint)?
>>> >
>>> > Regards,
>>> > Olaf
>>> >
>>> > [1] https://github.com/jaspervdj/blaze-markup/issues/36
>>> > _______________________________________________
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>>
>>
>>
>> --
>> -Andrew Thaddeus Martin
>>
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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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