[Haskell-cafe] How to write a Monad instance for this type

[Artem Pelenitsyn]

Being able to omit []'s and commas is a very natural desire, and it's not
clear why would anyone need to engage with monads to achieve this. Although
today monads is probably the only way (NB: look at the RebindableSyntax
extension), there were a number of discussions over GHC Proposals as to how
to retrofit it into the language, e.g. [Record with Syntax][Record-with].
This and (closed unmerged) the [Extra Commas][Extra-Commas] proposal
prompted Joachim to start a [wiki page][All-things-layout] gathering this
kind of ideas — which I highly recommend to check out and, possibly,
brainstorm. Hopefully, it will turn into a new, this time successful GHC
proposal.

[Extra-Commas]: https://github.com/ghc-proposals/ghc-proposals/pull/87
[Record-with]: https://github.com/ghc-proposals/ghc-proposals/pull/231
[All-things-layout]:
https://gitlab.haskell.org/ghc/ghc/wikis/All-things-layout

--
Kind regards, Artem

On Tue, 3 Sep 2019 at 13:42, MarLinn <monkleyon at gmail.com> wrote:

> Hi.
>
> I end up with this type.
>
> data Hdf5M a
>   = H5Root (Hdf5M a)
>   | H5Group ByteString [Hdf5M a]  -- A group can contain other groups and/or datasets
>   | forall sh b. (NativeType b, Shape sh) => H5Dataset ByteString (Array F sh b)
>
> […]
>
> And I would like to be able to describe a contain like this
>
> hdf5 $ do
>       group "name" $ do
>             dataset "name1" array1
>             dataset "name2" array2
>             group "other-name" $ do
>                      etc...
>
> You don't need a monad instance for this. First of all, you don't even
> need do syntax to make something "pretty" similar to this.
>
> 	hdf5 $
> 	    group "name" $
> 	        [ dataset "name1" array1
> 	        , dataset "name2" array2
> 	        , group "other-name" $ [
> 	            …
>
> But if you insist, you can just use an existing monad like WriterT. For
> example (simplified):
>
> 	data Composite a = Base a | Composite [Composite a]
> 	
> 	type CompositeWriter a = Writer [Composite a] ()
> 	
> 	base = tell . pure . Base
> 	
> 	composite = censor (pure . Composite)
> 	
> 	asComposite = Composite . snd . runWriter
> 	
> 	test = asComposite $ do
> 	    base 'a'
> 	    base 'b'
> 	    composite $ do
> 	      base 'c'
> 	      base 'd'
>
> Hope that helps.
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