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Assignment involving generic property of generic object fails to typecheck correctly within generic function


By : Aldrin Ilagan
Date : October 18 2020, 06:10 PM
This might help you The first option (using KeysOfPropertiesWithType) does not work because typescript can't reason about conditional types that still contains unresolved type parameters (such as T and K in this example)
The second option does not work because T extends Record means T can for example be { a: false } which would mean the assignment obj[key] = true would not be valid. Generally, the fact that T[K] must extends a type does not mean that inside the generic function we can assign any value to it, the constraint just tells us what the minimum requirement is for the value, we don't yet know the full contract T[K] requires.
code :
function booleanAssignmentTest2<K extends PropertyKey>(obj: Record<K, boolean>, key: K): void {
    let foo: boolean = obj[key]; // Fine!
    let foo2: string = obj[key]; // Error: working as intended!
    obj[key] = true; // Ok now we know T[K] is boolean
}

let foo = { aBool: false, aNumber: 33, anotherBool: false };

booleanAssignmentTest2(foo, "aBool"); // Fine!
booleanAssignmentTest2(foo, "anotherBool"); // Fine!
booleanAssignmentTest2(foo, "aNumber"); // Error: working as intended!
function booleanAssignmentTest2<T extends Record<K, boolean>, K extends keyof T>(obj: T, key: K): void {
    let foo: boolean = obj[key]; // Fine!
    let foo2: string = obj[key]; // Error: working as intended!
    obj[key] = true as T[K]; // ok now
}


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Generic class fails to accept object with direct line to the generic where type restriction


By : hobby
Date : March 29 2020, 07:55 AM
should help you out I have a c# data class defined with an inheritance chain that looks like this: , The following code seems to work for me.
code :
public class Screen { }
public abstract class EntityBaseCore
{    }

public abstract class EntityBase : EntityBaseCore
{    }
public partial class AdmSite : EntityBase
{    }

public abstract class ViewModelSecurityBase<T> : Screen where T : EntityBaseCore
{    }
public abstract class EditorViewModelBase<T> : ViewModelSecurityBase<T> where T : EntityBaseCore
{    }

public class SiteViewModel : EditorViewModelBase<AdmSite>
{    }

How to typecheck a generic function using a value of that same generic type in Flow


By : user1523679
Date : March 29 2020, 07:55 AM
I hope this helps you . for this very interesting question! Unfortunately I think that right now it is not possible to refine the type T by refining the type of the first element in the array. This can be seen when we use the following statement:
code :
if (firstElement) {
  if (typeof firstElement.id === 'string') {
    const refined: string = firstElement.id;
    const notRefined: T = refined; // Errors
    const alwaysWorks: T = firstElement.id;
  }
  const alwaysWorks: T = firstElement.id;
}

How can get TypeScript to correctly type check call to a generic function that in turns calls a function with a generic


By : user2419118
Date : March 29 2020, 07:55 AM
it should still fix some issue I am trying to write code very similar to this: , Lift T parameter to outer func.
code :
const a = (m: <T>() => T) => { 
  return m();
}
// Type 'string' is not assignable to type 'T'
a(() => 'Hey!');
const a = <T>(m: () => T) => { 
  return m();
}
// Ok!
a(() => 'Hey!');
const useSearchState = <T>(mapState?: (state: SearchState) => T) => {
  if (mapState) {
    //                       Type 'T' is not assignable to type 'HasSearchState'.   
    return useHasSearchState((hasState) => mapState(hasState.searchState));
  }
  return useHasSearchState((hasState) => hasState.searchState);
};
const useSearchState = <T extends HasSearchState>(mapState?: (state: SearchState) => T) => {
// Type 'string' is not assignable to type 'HasSearchState'.
const useId = () => useSearchState(state => state.id);

Constraint generic function by comparing instance property type with generic parameter


By : Bohao Cao
Date : March 29 2020, 07:55 AM
it helps some times What you are requesting is some kind of logical OR on type constraints:
code :
... where D == HasFirstDependency || HasSecondDependency

How do I create a generic Scala function that returns an object based on the generic type?


By : OMFSM
Date : March 29 2020, 07:55 AM
hop of those help? Take a look at Scala "manifests". They often allow you to circumvent type erasure on the JVM.
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