checker

That is a big word, but it boils down to this: let's say you wrote an identity function of type "a -> a", and it passed the type checker.

The ID checker, that's a job. The pat-down guy, that's a job. The guys that stand in front of the tablet app that digitally flips a coin to see if you get your hands swabbed and chromatographed, apparently that's three jobs.

Or someone at WikiLeaks checked for typos using a British-English spell checker and corrected a few US-English spellings?

By invalid, I simply mean that a parse error would be reported by a conformance checker whenever such constructs are found in a document.

I think it can be proven that if you write an implementation that passes the type checker for this signature, the implementation is necessarily correct.

For one, some sort of syntax or type checker is actually trying to understand your queries and makes it easy to find typos before the database laments in the middle of a huge transaction.

The type checker will warn you when you have a type declaration that appears to be incorrect, and will optimized based on these declarations, but will not keep you from running code that violates a type-check.

Objects are closed, so you can't accidentally add a new property because of a typo, and you get static warnings about undefined property names.\n * Dart has optional type annotations and a static type checker.

There's something so excruciating about acting as a hypothesis checker for hours while the people in the room gradually develop the knowledge they need to solve their own problems because they don't trust you enough to delegate.