Elide common lines in string diffs

[evanrelf]

When a long string differs between derivations - usually some long shell script written in a multi-line string - nix-diff prints the entire string contents, even if only a few lines of that long string have changed.

It would be nice if the parts in common were elided, like diff does with files.

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I've reproduced this in a gist to illustrate: https://gist.github.com/evanrelf/2b1b1772c5f83ca102c89d29e4839aa3

I have two derivations that write similar (long) shell scripts using writeShellScript. Only one line is different between the Nix source code and the resulting shell scripts.

diff -u a.nix b.nix and diff -u $(nix-build a.nix) $(nix-build b.nix) are both very short; they elide parts in common.

nix-diff $(nix-instantiate a.nix) $(nix-instantiate b.nix) prints the whole string.

[SuperSandro2000]

I am really interested in this feature and it is really the only bummer in nix-diff right now.

Maybe the diff algorithm should be replaced by something smarter in general like https://hackage.haskell.org/package/Diff ?

[Gabriella439]

I'd accept a pull request to fix this, but I likely won't have the time to implement this myself

[SuperSandro2000]

I tried getting that implemented but it turns out my none existing Haskell knowledge made it impossible.

[evanrelf]

@SuperSandro2000 I don't believe the Diff library is any more intelligent than the patience library nix-diff uses today. In fact, I replaced Diff with patience a while ago because it improved the diffs! 🙂

[SuperSandro2000]

Good to know that!