Well I don't literally mean finished as in you haven't finished typing it. Although that's possible too - whitespace languages like Python tend to just work since they don't have end brackets, I suppose.

But you could have a compiled language where errors were limited to the function when possible, like by emitting asserts.

there are approaches to ensure you always or almost always have syntactic validity, e.g. structured editing or error correcting parsing, though of course it's true that the more permissive the system is the more tortured some of the syntactic 'corrections' must be in extreme cases. the approach we're taking with http://hazel.org (which we approximate but don't fully succeed at yet) is: allow normal-ish typing, always correct the code enough to be ran/typechecked, but insert placeholders/decorations to telegraph the parse structure that we're using in case of errors or incompleteness.

Perhaps we are talking about programming with holes:

https://hazel.org/

They didn't say "syntactically invalid".