# thunderstrike.ai intern resources

## purescript
**Purescript** is a strongly-typed <a href="https://en.wikipedia.org/wiki/Type_safety"><sup>[type safety]</sup></a>
purely functional <a href="https://en.wikipedia.org/wiki/Purely_functional_programming"><sup>[purely functional]</sup></a>
programming language that compiles to javascript (like typescript).

### In brief
<ul>
  <li>
  effectful code
  <a href="https://en.wikipedia.org/wiki/Side_effect_(computer_science)"><sup>[side effect]</sup></a>
  is isolated from non-effectful code. This makes programs much more consistent to run and reason about.
  </li>
  <li>everything is an expression</li>
  <li>interacting with raw javascript is relatively painless</li>
</ul>

### Motivating example: React vs Halogen
this frontend app:
 1. renders `"Loading..."` until either data or error
 1. fetches data from a server
 1. if ok, render items in a list. if error, render error span at the bottom of the page.

**React**
```javascript
export const App = () => {
  const [items, setItems] = React.useState(undefined)
  const [error, setError] = React.useState(undefined)

  React.useEffect(() => {
    fetch('/api/item')
      .then(rep => rep.json())
      .then(items => setItems(items))
      .catch(e => setError(e))
  }, [])

  return error !== undefined
    ? (<p class="error">{error.message}</p>)
    : items === undefined
    ? (<p>Loading...</p>)
    : items.map(item => {
        return (
          <div key={item.id}>
            <p>{item.title}</p>
            <span class="small">{item.description}</span>
          </div>
        )
      })
}
```

**Halogen**
```haskell
type Item = {id :: String, title :: String, description :: String}
data State = StateEmpty | StateErrored Error | StateGotItems (Array Item)
data Action = Init

renderItem item = div [] [ p [] [text item.title]
                         , span [className "small"] [text item.description]
                         ]

renderApp StateEmpty = p [] [text "Loading..."]
renderApp (StateErrored e) = p [className "error"] [text (message e)]
renderApp (StateGotItems items) = map renderItem items

action Init = do
  response <- fetch (URL "/api/item") {method: Get}
  jsonString <- text response
  itemsOrError <- readJSON jsonString
  case itemsOrError of
    Right items -> put (StateGotItems items)
    Left error -> put (StateErrored error)

app = mkComponent
  { initialState: StateEmpty
  , render: renderApp
  , eval: mkEval (defaultEval {handleAction = action, initialize: Just Init})
  }
```