The author seems concerned about compile-time range checking: did you handle the full range of inputs?
Range checking can be very annoying to deal with if you take it too seriously. This comes up when writing a property testing framework. It's easy to generate test data that will cause out of memory errors - just pass in maximum-length strings everywhere. Your code accepts any string, right? That's what type signature says!
In practice, setting compile-time limits on string sizes for the inputs to every internal function would be unreasonable. When using dynamically allocated memory, the maximum input size is really a system property: how much memory does the system have? Limits on input sizes need to be set at system boundaries.
Range checking can be very annoying to deal with if you take it too seriously. This comes up when writing a property testing framework. It's easy to generate test data that will cause out of memory errors - just pass in maximum-length strings everywhere. Your code accepts any string, right? That's what type signature says!
In practice, setting compile-time limits on string sizes for the inputs to every internal function would be unreasonable. When using dynamically allocated memory, the maximum input size is really a system property: how much memory does the system have? Limits on input sizes need to be set at system boundaries.