optional type guard

[RebeccaStevens]

This adds the function tg.optional which takes a type guard as an argument and returns an optional version of that typegaurd.

This allows isOfShape to handle optional properties.

fix: #35
re: #36

Guard	Type
tg.isOfShape({ a: tg.optional(tg.isNumber) })	{ a?: number }
tg.isOfShape({ a: tg.isOneOf(tg.isNumber, tg.isUndefined) })	{ a: number | undefined }

[RebeccaStevens]

I'm not a big fan of using a symbol for identify "optional type guards" as it means only type guards that go through the "optional" function defined by this library are considered "optional type guards".

This symbol could possibly be exported so that others can use it but I think it would be best to remove it all together. However to remove it, an alternative implementation of isOptional or the part of isOfShape that uses isOptional would be needed.

[RebeccaStevens]

One possible solution would be to introduce a new type for Guards with options and allow this type to be used in a Shape. "optional" could then simply be an option for guards.

The type could possibly just be a tuple like this:

type GuardWithOptions<T> = [Guard<T>, GuardOptions]
type GuardOptions = {
  optional?: boolean
}

[lazarljubenovic]

I see the mentioned drawbacks with the symbol, but I think it's fine for this library. Something more robust (like a full-blown validation library) would probably benefit from a more complex approach; type-guards is supposed to be a quick utility to help with basic validation (“binary” validation I guess, since it's either a pass or fail, i.e. no specific errors/messages of why it failed).

[RebeccaStevens]

@lazarljubenovic Discovered a bug with regard to the typings.
The returned guard type of isOfShape still has the optional keys typed as key: type | undefined rather than key?: type.
Unfortunately we can't solve this issue with the same sort of symbol hack that was used to solve the actual implementation. Another new approach might be needed.

This whole optional problem thing is proving much harder than I thought it would be. 😞

[lazarljubenovic]

@RebeccaStevens Yeah, the problem makes me rage quit every time I have a stab at it 😄 This might help with what you mention but I had troubles wrapping my head around the whole thing in combination with type guards. Maybe you'll have more luck 😅