Optional types in Tuple

[sagro111]

Hi!

i try to type arrays items

I use tuple record
const tuple = t.Tuple(t.String, t.String.optional())

And when i try to check array like:
tuple.check(['Hello'])

I get error
Failed constraint check for [string, string | undefined]: Expected length 2, but was 1

I expect optional parameters to be ignored

[yuhr]

Optional runtype (which is generated by .optional() shorthand) doesn't work like that. As described in the error message, your tuple corresponds to [string, string | undefined] in TS's notation, that obviously doesn't accept ['Hello']. In such case you must use Union around two Tuples:

const tuple = t.Union(t.Tuple(t.String), t.Tuple(t.String, t.String))

We've noticed recently that the naming of Optional is somewhat misleading, so we're planning to have it reporting a static error to the user when misused.