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Understanding purescript-variant


Understanding purescript-variant

By : Blake Scott
Date : October 25 2020, 09:10 AM
it fixes the issue Both foo and bar restrict that the variant type has to contain some tag, but they don't prevent other tags from being included in the variant. The concrete types when you apply fooToString bar would be:
code :
 bar :: Variant (bar :: Boolean, foo :: Int) -- v = (foo :: Int)
 fooToString :: Variant (foo :: Int, bar :: Boolean) -> String -- v = (bar :: Boolean)
bar :: forall r1 r2. RowCons "bar" Boolean r1 r2 => Variant r2
fooToString :: forall r1 r2. RowCons "foo" Int r1 r2 => Variant r2 -> String
bar :: RowCons "bar" Boolean (foo :: Int) (foo :: Int, bar :: Boolean) => Variant (foo :: Int, bar :: Boolean)
fooToString :: RowCons "foo" Int (bar :: Boolean) (foo :: Int, bar :: Boolean) => Variant (foo :: Int, bar :: Boolean) -> String


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Understanding boost::variant

Understanding boost::variant


By : Sunil Shanawad
Date : March 29 2020, 07:55 AM
This might help you Overload resolution happens at compile time, when your boost::variant instance could contain either type, so the compiler has no way of knowing whether to call void f(int8_t) or void f(int16_t).
std::cout << a works because in either case it's calling the same function, std::ostream &operator<<(std::ostream &, const AttrValue &), which inside dispatches on the runtime type of the instance.
code :
struct f_visitor: public boost::static_visitor<void>
{
    template<typename T> void operator()(T t) const { return f(t); }
};

boost::apply_visitor(f_visitor(), a);
Understanding Purescript Eff Monad and do blocks

Understanding Purescript Eff Monad and do blocks


By : user6358861
Date : March 29 2020, 07:55 AM
wish helps you If you're working with version 0.12.0 of halogen you should be able to use fromEff from https://pursuit.purescript.org/packages/purescript-aff-free/3.0.0/docs/Control.Monad.Aff.Free#v:fromEff like so:
code :
data Query a = ToggleState a

eval :: Query ~> H.ComponentDSL State Query g
eval (Toggle next) = do
    H.modify (\state -> state { isOn = not state.isOn })
    _ <- H.fromEff (log "updating the state!")
    pure next
How to combine rows of record types in PureScript? (Is there any alternative to the Union typeclass in PureScript 0.12.0

How to combine rows of record types in PureScript? (Is there any alternative to the Union typeclass in PureScript 0.12.0


By : Kuljeet Kaur
Date : March 29 2020, 07:55 AM
I wish this helpful for you Problem: I have different record types with many common fields. How could I "include" the common fields in the record type definitions? , PureScript has a special syntax for combining records:
code :
type Common = ( a :: Int, b :: Int )
type Record1 = { y :: String | Common }
type Record2 = { z :: Boolean | Common }
newtype RecordType3 = RecordType3 { w :: Number | Common }
type CommonRec = Record Common 
-- equivalent to:  CommonRec = { a :: Int, b :: Int }
type CommonPlusFoo = ( foo :: Bar | Common )
type RecWithFoo = { x :: Int | CommonPlusFoo }
type Record1Poly r = { y :: String | r }
type Record1 = Record1Poly Common
updateName :: forall r. { name :: String | r } -> { name :: String | r }
updateName x = x { name = "Mr. " <> x.name }

jones = { name: "Jones", occupation: "Plumber" }
mrJones = updateName jones  -- mrJones = { name: "Mr. Jones", occupation: "Plumber" }
type Record1Poly r = { y :: String | r }
type Record1 = Record1Poly Common
type OnlyY = Record1Poly ()
 x = RecordType3 { w: 42.0, a: 1, b: 2 }
 y = w x
 (RecordType3 k) = x
 y = k.w
 unRecordType3 (RecordType3 k) = k
 y = (unRecordType3 x).w
What's a a tuple variant? (understanding compiler error message)

What's a a tuple variant? (understanding compiler error message)


By : Giovanni Farias
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , the answer was provided as a comment. Ok is spelled with lower-case k but it didn't explain the meaning of error message.
First, here is the working code with that one small change:
code :
#[wasm_bindgen]
pub fn hello_hash(count: i32) -> Result<JsValue, JsValue> {
    set_panic_hook();
    let mut book_reviews = HashMap::new();

    book_reviews.insert(
        "Grimms' Fairy Tales".to_string(),
        "Masterpiece.".to_string(),
    );
    let data = BookStoreData {
        h: book_reviews,
        name: "My Book Store".to_string(),
    };

    let js_result: JsValue = JsValue::from_serde(&data).unwrap();

    Ok(js_result)    

}
struct S(usize); // tuple-like struct
enum E {
    T(usize), // tuple variatn
}
Purescript, understanding why no type class instance was found

Purescript, understanding why no type class instance was found


By : SVS Aditya
Date : March 29 2020, 07:55 AM
will help you There is no MonadEff instance for Free, so you cannot use liftEff in ComponentDSL, as it is a synonym for a Free.
This is changing in the next Halogen release, and ComponentDSL will have a MonadEff instance. For the currently released version (v0.12.0) the solution is to use the fromEff function instead, which is mentioned in the "Non-state effects" section of the guide.
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