Add cotransverse

[Lysxia]

Some kind of dual to transverse.

For an example, this can be used to define thing here: https://www.reddit.com/r/haskell/comments/aj2xwn/is_there_something_deeper_in_this_stateful_tree/

[phadej]

So obvious when you see it, great!

[Lysxia]

I need to fix the property, distribute is the wrong way around...

[phadej]

I took liberty to add few examples in f05582d (EDIT: and following typo fix)

[gelisam]

I took liberty to add few examples in f05582d

I'm surprised to see the $setup stanza, does that mean we have a doctest test suite somewhere I haven't noticed? If not, let's add one :)

[phadej]

@gelisam I use doctest Data/Functor/Foldable.hs locally. We can add something like that to Travis-CI. Alternative is to make a proper test-suite, but it would currently require changing to build-type: Custom, which I largely dislike.

[gelisam]

Alternative is to make a proper test-suite, but it would currently require changing to build-type: Custom

Which part requires build-type: Custom? I don't see this requirement in the documentation of doctest nor doctest-discover.

[phadej]

Which part requires build-type: Custom? I don't see this requirement in the documentation of doctest nor doctest-discover.

doctest executable doesn't require even a package to exist, only source files somewhere. But in package test-suite case, we need to provide doctest right package environment (dependencies), paths (source files, autogenerated ones), etc (e.g. default-extensions) flags. Historically, doctest was created when all dependencies where in a global package database. It's not true with cabal new-build or stack.

The iterations of Edward's custom Setup.hs lead to creation of https://github.com/phadej/cabal-doctest, so we don't need to copy&paste Setup.hs scripts. Sadly seems we are missing a motivation/introduction from its README.

Without custom-setup you'll get headache making doctest test-suite work with cabal-install, stack, nix derivations etc. ekmett/lens#701 was a ~starting point to cabal-doctest library.

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TL;DR the current approach, doctest running in CI relying on .ghc.environment.* files is simplest (even there I cheat a little so we don't need to look for Paths_recursion_schemes.hs); the downside is that /Stackage/ won't run them, but it's IMHO a small price to pay.

Alternative is to use cabal-doctest, but I'd rather not turn any package to use build-type: Custom at this point.