What is an efficient way of loop through rows of a polars LazyFrame?

I have a large Parquet file with just a few columns, and I'd like apply a function to each row of the Paruqet file. What is an efficient way of doing that? By efficient, I mean 2 things.

1. The code runs fast.
2. The code doesn't load the while file into memory but rather process it a few rows at a time.