Extended List module

[wojpok]

This code is part of a pretty-printer PR #256 . This code have been separated from main PR for easier review and merging.

[ppolesiuk]

The invariant that dropLastAux works on non-empty list can be easily ensured by passing the head and the tail of the list separately. In my opinion such an implementation is more clear and doesn't use suspicious impossible function. Same comment applies to other last-related functions, as well as foldRight1* functions.

Suggested change

	let rec dropLastAux xs =
	match xs with
	| [] => impossible ()
	| [x] => []
	| x :: xs => x :: dropLastAux xs
	end in
	if isEmpty xs then
	None
	else
	Some (dropLastAux xs)
	let rec dropLastAux y xs =
	match xs with
	| [] => []
	| x :: xs => y :: dropLastAux x xs
	end in
	match xs with
	| [] => None
	| x :: xs => Some (dropLastAux x xs)
	end

[wojpok]

Cool trick. I will apply that

[ppolesiuk]

Now, the code looks good. I propose some minor fixes in documentation comments, and I would prefer to be consistent in the order of pattern-matching clauses: some functions (like last) match with the empty list first, but the others (e.g., dropLast) put this clause as the last one. If these things will be fixed, I will opt for merging this PR.

[ppolesiuk]

None could be quoted, to be consistent with previous documentation comments.

Suggested change

	Returns the last element of a list or None if the list is empty.
	Returns the last element of a list or `None` if the list is empty.

[ppolesiuk]

Suggested change

	`foldLeft f [x1, x2, ... ,xn]` is `f (... (f (f x1 x2) x3) ...) xn`.
	`foldLeft1 f [x1, x2, ... ,xn]` is `f (... (f (f x1 x2) x3) ...) xn`.

[ppolesiuk]

Suggested change

	`foldLeft f [x1, x2, ... ,xn]` is `f (... (f (f x1 x2) x3) ...) xn`.
	`foldLeft1Err f [x1, x2, ... ,xn]` is `f (... (f (f x1 x2) x3) ...) xn`.

[ppolesiuk]

Suggested change

	`foldRight f [x1, x2, ..., xn]` is `f x1 (... (f x(n-1) xn)))`.
	`foldRight1 f [x1, x2, ..., xn]` is `f x1 (... (f x(n-1) xn)))`.

[ppolesiuk]

Suggested change

	`foldRight f [x1, x2, ..., xn]` is `f x1 (... (f x(n-1) xn)))`.
	`foldRight1Err f [x1, x2, ..., xn]` is `f x1 (... (f x(n-1) xn)))`.

[wojpok]

All pattern matching begins now with an empty list

[ppolesiuk]

Looks good. Thanks!

[Copilot]

Pull Request Overview

This PR extends the List module with new functions for operating on the last elements of lists and for folding operations that don't require an initial accumulator. The changes introduce functions with both Option-based and error-handling variants.

• Adds functions for operating on the last element of lists (dropLast, last, dropTakeLast)
• Introduces fold operations without initial values (foldLeft1, foldRight1)
• Provides both Option-returning and error-handling (Err) variants for all new functions

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/List.fram	Implements new list functions and exposes them as methods on lists
test/stdlib/stdlib0003_List.fram	Adds comprehensive test coverage for all new list functions

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