[Haskell-cafe] do-notation for building monoid values

[Andrew Martin]

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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