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

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 wrote:

I do the same thing that Merijn does. It works really well.

On Thu, Mar 8, 2018 at 8:08 AM, Merijn Verstraaten

...

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 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 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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