object() decoder needs a 'strict' option to disallow undefined fields about json-type-validation

[tohagan]

Actually I think there a more functional approach that you'd probably recommend.
We just add a strictObject(decoders) function that invokes object(decoders).

Here's my untested JavaScript (not TypeScript) version

const checkStrict = (decoders) => {
  return new Decoder((json) => {
    for (const key in json) {
      if (!decoders.hasOwnProperty(key)) {
        return Result.err({message: `an undefined key '${key}' is present in the object`});
      }
    }
    return Result.ok(json);
  });
}

// like object() but checks that only declared fields are used.
export const strictObject = (decoders) => object(decoders).andThen(checkStrict(decoders));

[tohagan]

Alas .. That won't work because you've made the Decoder() constructor private.

[psiservices-rcocks]

I'm also looking for such a feature. In keeping with the library perhaps there could be an only combinator, so a decoder could be defined as:

const decoder = only(object({a: string()})

where this decoder would throw on {a: "hello", b: "world"} but not {a: "hello world"}.