setter-getter-2.ts

interface SetGet<A,V> {
  setValue: (o:A, v:V) => A;
  getValue: (o:A) => V;
}


function f<A>(k:keyof A):SetGet<A, A[typeof k]> {
  return {
    setValue: (a:A, v:A[typeof k]) => Object.assign({}, a, {[k]: v}),
    getValue: (a:A) => a[k]
  }; 
}

interface B {
  s: string;
  n: number;
}

function f2<A>(a:A, sg:SetGet<A,string>):string {
  const a2 = sg.setValue(a, 'aa');
  return sg.getValue(a2);
}

const bs = f<B>('s');
const bn = f<B>('n');
const b:B = {s:'s',n:0};
f2(b, bs);
f2(b, bn);
/*
  
  foo.ts:27:7 - error TS2345: Argument of type 'SetGet<B, string | number>' is not assignable to parameter of type 'SetGet<B, string>'.
  Type 'string | number' is not assignable to type 'string'.
    Type 'number' is not assignable to type 'string'.

27 f2(b, bs);
         ~~


foo.ts:28:7 - error TS2345: Argument of type 'SetGet<B, string | number>' is not assignable to parameter of type 'SetGet<B, string>'.

28 f2(b, bn);
         ~~

*/