Uncovering the Magic of TypeScript Type Inference with Generics and Function Objects

As I attempted to create a versatile function that can accept an interface containing both functions and data, there seems to be an issue with inference. Assistance in resolving this problem would be greatly appreciated.

To view the code that is failing to compile, please visit this link to the CodeSandbox.

function InitializeModel<S, C, M>(params: {
  state: S;
  computed: (s: S) => C;
  methods: (s: S, c: C) => M;
}) {
  const state = params.state;
  const computed = params.computed(state);
  const methods = params.methods(state, computed);
  return {
    state,
    computed,
    methods
  };
}

export const myModel = InitializeModel({
  state: { message: "Expecting Correct Inference Here" },
  computed: (state /* inference should work */) => ({
    computedMessage: state.message + "But This Won't"
  }),
  methods: (state /* inference should work */, computed /* this one was inferred incorrectly */) => {
    return {
      logName: () => console.log(state.message),
      logComputedName: () => console.log(computed.computedMessage) // Compilation Error
    };
  }
});

Answer №1

It appears that the current version of Typescript does not support this functionality.

After conducting some tests with your code, I have discovered that Type Inference works with a specific internal priority. This priority dictates that the type should be inferred from the parameter whenever possible, rather than from the return value.

If you remove the methods parameter from your code, you will notice that the computed return value - C, will be correctly inferred as:

{ computedMessage: string }

However, when the methods parameter is included, C is inferred as unknown. This is because it exists as a parameter in methods, causing Typescript to prioritize determining the correct type based on the behavior of methods rather than computed.

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