In the following clip at 19:00 Brave new world he shows an example that looks like the following Record (Persistable Payment | Callbackable | r). Whatever I try I am not able to reproduce the syntax in a simple example. The only way I understand how to use rowtype is type MyType = forall r. {a::String | r}.
Can someone please explain how his example works with a simpler example and why am I not able to use a syntax like Record (Int | String | r)?
Hi,
the syntax with the curly braces { name :: String, age :: Int } is syntactic sugar for Record ( name :: String, age :: Int) (note that you still need the labels)
so something like this should work:
type MyRec r = Record ( name :: String, age :: Int | r)
test :: forall r . Record ( name :: String, age :: Int | r) -> String
test r = r.name <> show r.age
res = test { name: "Dude", age: 42 }
There is a bit more about this in the docs
And there is a great section on more advanced usages of recors in Jordan’s Reference
Maybe this will help you further - if not feel free to ask 
PS in the example from the video the labels are in Persistable and the other parts:
type Persistable a =
(persist :: a -> Aff Unit)
He seems to be using these as building-blocks/parts (with |) to put together records with fields from all those - to be honest I think I never used this (I love this but I never actually had the need) so I’m not sure but AFAIK it should still work (I think you’ll have to replace | with ‘+’ from RowApply?) more or less like this (there was a few changes to purescript and it’s syntax)
Ok it is probably that the syntax has changed then why I cant understand what he is doing. Thanks for the reply!
I’m not 100% sure that | will not work (I did not try and I don’t actually know the syntax rules for these) - I think this does only work if left of the | is a "list of label :: type pairs though.
Can you share the code you tried? Maybe we can work it out together.
What I have tried is from the example from the clip.
type Persistable a = (persist :: a -> Aff Unit)
tt :: forall r. Record (Persistable Int | String | r)
tt = ??? -- BTW is there a way in purescript to have an unimplemented function type check?
However this just gives me syntax error at the first |
There is undefined from the Undefined Package - but for just setting up something like this will work too
undefined :: forall a. a
undefined = go unit
where go unit = go unit
And yes it seems you can do
tt :: forall r. Record ( label :: String | Persistable Int )
but not add | r
Once you import type (+) and change your Persistable a bit it works:
import Type.Row (type (+))
type Persistable a r = (persist :: a -> Unit | r)
tt :: forall r. Record ( label :: String | Persistable Int + r)
tt = undefined
this type-checks for me.
Of course this is a bit silly as + r is a bit like x + 0 in common arithmetic - so you could do
type Persistable a r = (persist :: a -> Unit | r)
tt :: forall r. Record ( label :: String | Persistable Int r )
tt = undefined
here too - but once you have more parts I’d go with +
PS: Don’t forget the label names
PPS: I guess you could generalize my last example there to Record ( moreLabals | ExtensionOne (ExtensionTwo (... ExtensionN r)...)) ) as was hinted in Jordan’s Reference.
AFAIK, that’s not valid syntax. There were some major parser changes made in v0.13.0 which was released in May 2019. So, perhaps these guys were on a v0.12.x release?