PureScript Meetup and some Haskell

So far, so good

• I have an easy exam today, and two upcoming phone screeners. So far, so good. It hasn’t been a week yet, so pretty exciting.

Watching a webinar on Asynchronous Exceptions in Haskell

And some Notes from LA PureScript

• Phil’s talk
• purescript incremental - avoid computing unnecessarily
• compute changes in outputs from changes in inputs
• idea from paper “A Theory of Changes for Higher Order Languages”. Pdf available via arxiv here.
• change structures: monoid of changes which act on carrier types (transitively)

class Monoid da <= Patch a da
    | a -> da
where
    patch :: a -> da -> a

class Patch a da <= Diff a da
    | a -> da
where
    diff :: a -> a -> da

• purescript-incremental
  • uses higher order abstract syntax
  • inspired by automatic differentiation

Jet functions
a -> b -> da -> db

f :: Jet a da -> Jet b db

• incremental dom : not having to recreate DOM every time
• translate change DOM to DOM:
  • implemental in the database
  • incremental parsing
  • incremental canvas graphics
• libraries that correlate: purescript-sdom and purescript-panda
• sdom

• panda

• M. Trotter (purescript-react-basic).
• Michael was visiting the office from Utah. He also runs a Haskell group. You can attend their Google Hangouts and learn/write some Haskell here.

component :: ReactComponent ExampleProps
component = react
{
displayName : "Counter"
, initialState : {counter...}
}

_ _ _ are three arguments that correspond to props, state, set state

component::ReactComponent{}
R.dir{children: [createElement
ToggleButton
component
{on:true}

• T.Honeyman
• Designing Flexible Open-Source Components in Halogen
• UI components inflexible because they handle rendering.
• their corresponding repo is purescript-halogen-select (see here)

let action = onClick $ action $ Select.Raise
true -> [action class_ "highlighted"]
false -> [action]

• functioning component you can add to depending on your needs
• SetProps functions are event Handlers in Halogen
• Handle output messages a ~> case message of
  • Emit
  • Searched Search
  • Selected Item
• Conclusion: We should design more components without render functions
• Advantages:
  • Small, simple APIs
  • Complete Design Freedom
  • Flexible & Extensible Functionality
  • new structure can be made in parent component and passed to child
• Disadvantages:
  • the user has to write their own rendering code
  • the library must impose some structure (even if minimal) eg. select still has to have an item to be selected
  • this ability may not be good for larger amounts of complexity (outputs that return other outputs, etc)
  • the library author does not know the structure of the user’s HTML
• Central idea; focus on behaviors, not renderers

• Inspiration (one of): Downshift by Paypal.

• Extensible Checked Exceptions with Polymorphic Variants (Nate)
• Just a heads up, it takes me a while to process Nate’s talks, so I’m just going to write my notes down for this.
• Re-read in a few years, see if it’s clearer then. Guy has a deep knowledge of some concepts that I can’t currently grasp

get 
:: forall m
. MonadHTTP m
=> String
=> Except HTTPError m String

• string or raiseError

write
:: forall m
. MonadFs m
=> Path
-> String
-> ExceptT FsError m Unit

• extend with new functionality without breaking old code
• solution: use Polymorphic Variant
• record is a structural type
• Exceptions using variant

type HTTPError r=
(
type HTTPServerError :: String
| r
)
r -> fill in w/ extension of shape

• inj lifts/ associates value with row

• to work together, must use +r <- open row

• advantage: inference
• how handle exceptions
• once row is handled, eliminate type
• how: rows of handlers to list
• Union -> LHS + RHS to infer output

Except V excIn m a
Except V excOut m a (infer output)

• Exhaustive handle

• r becomes closed row of nothing

• repo: purescript-checked-exceptions

Okey doke

• I’m going to read that paper and get some clarity on Phil’s idea. I’d like to understand the Tags in Nate’s idea a bit more.
• Back to Haskell.

And..that’s it.