A spell checker isn't something in a binary state of working or not working. Its something that has the almost impossible task of working out what the user want

A spell checker isn't something in a binary state of working or not working. Its something that has the almost impossible task of working out what the user wanted and not what they asked for. Maybe most of your problems are a single letter mistake or a keyboard slip up but where good spell checkers shine is they know what you want even when you are miles off. I find googles spell checking to be exceptional at understanding the mapping between how a word sounds like and what it actually is even when they share very few letters in common. An easy example of what I mean is if the input is "shivon" and the spell checker is able to correct this to "Siobhán" because it knows this is how users try to spell it when they have no idea. A simple algorithm isn't able to do this because there are no logical rules of english to follow here, you would likely need a massive amount of user data to train on to solve this test case.