What is the method for defining specific requirements for a generic type's implementation?

Question

What is the method for defining specific requirements for a generic type's implementation?

I am facing an issue with the following code snippet, where I am trying to restrict the pairing of Chart objects based on correct types for the data and options objects. However, despite my efforts, the TypeScript compiler is not throwing an error in the specific scenario I want to prevent.

class Chart<TData extends ChartData<TOptions>, TOptions extends ChartOptions> { 
    public data?: TData;
    public options?: TOptions;
}

interface ChartData<TOptions extends ChartOptions> { }
interface ChartOptions { }

interface BarChartData extends ChartData<BarChartOptions> { barChartData: number }
interface BarChartOptions extends ChartOptions { barChartOption: string }

interface PieChartData extends ChartData<PieChartOptions> { pieChartData: number }
interface PieChartOptions extends ChartOptions { pieChartOption: string}

const barChart = new Chart<BarChartData, BarChartOptions>();
const pieChart = new Chart<PieChartData, PieChartOptions>();

const mixedChart = new Chart<BarChartData, PieChartOptions>(); // expected compilation error

// Here is the case I want to avoid:
mixedChart.data = { barChartData: 7 } as BarChartData;
mixedChart.options = {pieChartOption: 'hello' } as PieChartOptions

// The mixing of BarChartData and PieChartOptions should be prevented

I need the ability to update options and data separately due to implementation requirements, which is why ChartData does not have a property for options. Adding options: TOptions to ChartData would trigger the error, suggesting that not using TOptions in ChartData is causing the constraint to be ignored. How can I maintain this constraint without adding an unwanted/exposed property?

typescript

Answer 1

Answer №1

I managed to get everything working by introducing the type?: ChartType property to the base class and utilizing it as the type parameter. This approach turned out to be successful because the types for each instance of type in the implemented interfaces ended up being bar | undefined and pie | undefined, which ultimately proved to be incompatible with each other.

The significance of ensuring that these types matched became evident once type?: TChartType was incorporated into the chart-specific interfaces. Unlike my initial attempt of using TOptions, this solution is benign whether set or left unset, and is much less likely to cause confusion among users.

class Chart<TChartType extends ChartType, TData extends ChartData<TChartType>, TOptions extends ChartOptions<TChartType>> { 
  public data?: TData;
  public options?: TOptions;
  type?: TChartType;
}

export type ChartType = 'bar' | 'pie' | 'line';

interface ChartData<TChartType extends ChartType> {
  type?: TChartType; // optional so that invokers don't actually have to set it
}

interface ChartOptions<TChartType extends ChartType> {
  type?: TChartType;
}

interface BarChartData extends ChartData<'bar'> { barChartData: number }
interface BarChartOptions extends ChartOptions<'bar'> { barChartOption: string }

interface PieChartData extends ChartData<'pie'> { pieChartData: number }
interface PieChartOptions extends ChartOptions<'pie'> { pieChartOption: string}

const barChart = new Chart<'bar', BarChartData, BarChartOptions>();
barChart.data = { barChartData: 7 }; // 'data' prop type is well-defined, 'type' prop can be ignored since that's just serving as a compiler hint

const mixedChart = new Chart<'bar', BarChartData, PieChartOptions>(); // error because PieChartOptions can't go with Bar charts 👍

Answer 2

I managed to get everything working by introducing the type?: ChartType property to the base class and utilizing it as the type parameter. This approach turned out to be successful because the types for each instance of type in the implemented interfaces ended up being bar | undefined and pie | undefined, which ultimately proved to be incompatible with each other.

The significance of ensuring that these types matched became evident once type?: TChartType was incorporated into the chart-specific interfaces. Unlike my initial attempt of using TOptions, this solution is benign whether set or left unset, and is much less likely to cause confusion among users.

class Chart<TChartType extends ChartType, TData extends ChartData<TChartType>, TOptions extends ChartOptions<TChartType>> { 
  public data?: TData;
  public options?: TOptions;
  type?: TChartType;
}

export type ChartType = 'bar' | 'pie' | 'line';

interface ChartData<TChartType extends ChartType> {
  type?: TChartType; // optional so that invokers don't actually have to set it
}

interface ChartOptions<TChartType extends ChartType> {
  type?: TChartType;
}

interface BarChartData extends ChartData<'bar'> { barChartData: number }
interface BarChartOptions extends ChartOptions<'bar'> { barChartOption: string }

interface PieChartData extends ChartData<'pie'> { pieChartData: number }
interface PieChartOptions extends ChartOptions<'pie'> { pieChartOption: string}

const barChart = new Chart<'bar', BarChartData, BarChartOptions>();
barChart.data = { barChartData: 7 }; // 'data' prop type is well-defined, 'type' prop can be ignored since that's just serving as a compiler hint

const mixedChart = new Chart<'bar', BarChartData, PieChartOptions>(); // error because PieChartOptions can't go with Bar charts 👍

What is the method for defining specific requirements for a generic type's implementation?

Answer №1

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