TDDwI: Minor changes to chapter10

[sagehane]

I hope this is the right place to suggest changes to https://idris2.readthedocs.io/en/latest/typedd/typedd.html.

While following the TypeDD book with idris2, I noticed that the code can be made total whilst resembling the original code more.

I checked the code with idris2 --check tests/typedd-book/chapter10/IsSuffix.idr but didn't run any tests because I didn't really understand how it works.

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Unrelated, but I feel like it would be nicer if the Snoc constructor was changed from:

Snoc : (x : a) -> (xs : List a) -> SnocList xs -> SnocList (xs ++ [x])

to something better resembling idris1:

Snoc : {x : _} -> {xs : _} -> SnocList xs -> SnocList (xs ++ [x])

I assume there is a valid motivation for this change that I didn't catch, as I've never used Idris outside of the purposes of this book.

[sagehane]

The comment probably can be improved. I'm especially confused as to why the new code doesn't need a | ysrec at line 300.

[sagehane]

Also, assuming this is the right place to have this file edited, the link at

Idris2/docs/source/typedd/typedd.rst

Lines 10 to 11 in ecd6d47

	code is also [going to be] part of the test suite (see `tests/typedd-book
	<https://github.com/edwinb/Idris2/tree/master/tests/typedd-book>`_ in the Idris

should probably be updated to this repo, instead of edwinb's repo.