TODO inline the problem statements
A linewise problem, so we can focus attention to just 1 line.
I use a parser combinator to find the first matching numeral. Handy that it's just built in to Lean, but I read the code and it's so simple, just like implementing one from scratch.
Their bottom-up nature appeals to my problem solving style.
To get the last matching numeral, I reverse the input string and then reverse the strings the parser is seeking.
Lean has a truly amazing amount of syntactic sugar. Unprecedented really. I thought I had become jaded about syntax but wow.
I really like the local "mutability". It let me write the code in a way that just ran without being perfect the first time. Then I cleaned it up.
I learned from Rust that local mutability is fine, glad to see this embrace.
Locally adding functions to types (like String.toDigit) allows the very useful extended dot notation.
Named and default arguments and partial application play together nicely.