Compile merging Optionals correctly in dhall-nix

[DarkKirb]

This is done by checking the type of the merge input, and emitting special code for Optionals.

This commit fixes #2443.

Please let me know if i did something poorly, i do not know haskell.

[Gabriella439]

Sorry for the long delay in responding to this.

I think there is a simpler approach that doesn't require threading through the context or inferring the type of any expressions. Rather, what you can do is to check if the argument to the Merge expression is of the form Some _ or App None _ and then change the translation. This makes use of the fact that the expression passed to loop is already pre-normalized so if it's an Optional value then it can only be one of those two cases

I can also implement my suggestion myself if you want, especially given how delayed my review was

[DarkKirb]

#2443 is only an issue for merges where the expression cannot be reduced to a form merge {Some = _, None = _} [Some _|None _].

some cases like λ(a: Natural) → merge { None = 0, Some = λ(a: Natural) → a } (Some a) compiles correctly, however λ(a: {None: Natural, Some: Natural → Natural }) → merge a (Some 1) also results in the nonsensical a: 1 a (i think that would be fixed with your suggestion though), the original test case wouldn’t be however

[Gabriella439]

What about in the general case translating the merge keyword to something like:

merge handlers unionLiteral

=>

if builtins.attrNames handlers == [ "Some", "None" ]
then … special case for Optional …
else … normal case …