How to handle this when this is a function #22285

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2. ghost added

   Needs More InfoThe issue still hasn't been fully clarifiedThe issue still hasn't been fully clarified

   on Mar 2, 2018
3. dagda1 commented on Mar 2, 2018

   

   Paul (dagda1)

   on Mar 2, 2018

   Last edited by dagda1

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   Andy (Andrewkraft) (@Andy-MS)

   Ok so here is a jsfiddle of plain js and here is the code:

   function extend(Class) {
     const name = Class.name;
   
     const symbol = Symbol(name);
   
     // callable `this` in  `Adder#add`
     Class.for = function _for(value) {
       return value[symbol];
     }
   
     Class.instance = function(constructor, methods) {
       constructor.prototype[symbol] = methods;
     }
   
     Class.symbol = symbol;
   
     const properties = Object.getOwnPropertyDescriptors(Class.prototype);
   
     Object.keys(properties)
       .filter(key => key !== 'constructor')
       .forEach(key => {
         // bind `for` so `this(a)` calls `Class.for` 
         Class.prototype[key] = Class.prototype[key].bind(Class.for)
       });
   
     return Class
   }
   
   const Adder = extend(
     class Adder {
       add(left, right) {
         const {
           add
         } = this(left);
         return add(left, right);
       }
     }
   )
   
   const {
     add
   } = Adder.prototype;
   
   Adder.instance(Number, {
     add(left, right) {
       return left + right;
     }
   });
   
   console.log(add(1, 2)); // 3
   
   Adder.instance(String, {
     add(left, right) {
       return left.concat(right);
     }
   });
   
   console.log(add("1", "2")); // "12"
   

   Class.for is what is is what gets called via this in this code

         const {
           add
         } = this(left);
   

   This will return the correct add dependant if it is a String or a Number.

   This is the code that makes this an alias to Class.for

     Object.keys(properties)
       .filter(key => key !== 'constructor')
       .forEach(key => {
         // bind for so `this()` calls `Class.for` 
         Class.prototype[key] = Class.prototype[key].bind(Class.for)
       });
   

   So back to the typescript example:

   it would be:

   export interface Adder<A> {
     add(left: A, right: A) => A;
   }
   
   const Adder = extend(
     class Adder<A> {
        add(this: (a: A) => Adder<A>, left: A, right: A): A {
          const {add} = this(left);  // will call Class.for for the specific implementation
          
          return add(left, right);
        }
     }
   );
   
   export { add } = Adder.prototype;
   

   So is this too exotic for typescript to recognise or can I somehow indicate that the this in { add } has this bound by the call in extend with the line Class.prototype[key] = Class.prototype[key].bind(Class.for)
5. dagda1 commented on Mar 3, 2018

   

   Paul (dagda1)

   on Mar 3, 2018

   Last edited by dagda1

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   This is really great but one thing, typescript does currently not support a symbol as an indexer:

   So the only way is to give the member any as its type

   export class TypeClass<A> {
     readonly symbol: any;
   

   Or is there another way?
7. dagda1 commented on Mar 3, 2018

   

   Paul (dagda1)

   on Mar 3, 2018

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   Andy (Andrewkraft) (@Andy-MS) this has been extremely helpful. I'm going to close the issue but is it possible to explain this commented line:

       instance<J extends T>(type: Function, implementation: J) {
           type.prototype[this.symbol] = implementation;
       }
   

   Why is it not just T that is the generic argument?
8. Paul (dagda1)

   closed this as completedon Mar 4, 2018
10. dagda1 commented on Mar 5, 2018

    

    Paul (dagda1)

    on Mar 5, 2018

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    Andy (Andrewkraft) (@Andy-MS) one problem with this is approach is that it expects the first argument to be the type argument, i.e.

    return ((left: any, ...args: any[]) => {
                const impl = left[this.symbol] as T;
    

    but what if the type in question is not the first argument in question. Generic types do not exist at runtime in typescript as I've no idea what that would compile to.

    I guess I would need to pass something into the instance function to indicate what where to get the type from?
12. microsoft

    locked and limited conversation to collaborators on Jul 25, 2018