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
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. _______________________________________________ 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.