TreeError.append is not associative - thus, the corresponding Validate is not an applicative functor

[david-christiansen]

See this comment: https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/Functional.20Programming.20in.20Lean

The exercises in the book should not have readers breaking the law - the exercise should be replaced with a better one!

[AaronNGray]

@david-christiansen Can I please ask the status of the source code for the book as I desparately need to be able to render it into PDF format for printing for my own purposes of learning LEAN.

[david-christiansen]

I'm not sure what you mean - this is the issue tracker for the source code repository for the book. Can you be more specific?