Suggestion: use refined input type instead of `unknown`

🐛 Bug report

Current Behavior

Types like NumberFromString currently allow decoding from any type (unknown). This can be useful but also error prone. For example, we might accidentally pass in a value which is already a number. We won't get a type error because decode is asking for unknown, which number is assignable to.

NumberFromString.decode(100) // no type error, returns a Left

Expected behavior

Reproducible example

Suggested solution(s)

If we changed the input from unknown to string, then decode would only work with strings. This way, mistakes like the one above would result in helpful type errors at compile time:

NumberFromString.decode(100) // type error!

NumberFromString.decode('100') // no type error, returns a Right

Implementation:

export interface NumberFromStringC extends t.Type<number, string, string> {}
export const NumberFromString: NumberFromStringC = new t.Type<number, string, string>(
  'NumberFromString',
  t.number.is,
  (u, c) => {
    const n = +u;
    return isNaN(n) || u.trim() === '' ? t.failure(u, c) : t.success(n);
  },
  String,
);

This is how I would expect the type to behave based on its name ("from string").

If the user needs to decode a value of type unknown, they can use t.string.pipe(NumberFromString) instead of NumberFromString.

WDYT? If you agree I will happily send a PR!

Additional context

Your environment

Which versions of io-ts-types are affected by this issue? Did this work in previous versions of io-ts-types?

Software	Version(s)
fp-ts
io-ts
io-ts-types
TypeScript

gcanti / io-ts-types