dhall-nix: Fix field-based union access

[ocharles]

Fixes #906.

I copied the approach used in dhall-json.

[Gabriella439]

If I understand correctly, this compiles < Foo >.Foo to { Foo }: Foo, but I believe the result should instead be: x: { Foo }: Foo x

[ocharles]

<Foo>.Foo compiling to {Foo}: Foo seems correct to me, and is the current behavior. What is that x about?

[Gabriella439]

Oh, you're right. Never mind, my thoughts were confused

[Gabriella439]

Now I remember what led to my confusion. The issue here is that you need to check if the alternative is empty when compiling the union to Nix.

For example:

dhallToNix "< Foo >.Foo" = { Foo }: Foo

dhallToNix "< Foo : {} >.Foo {=}" = { Foo }: Foo {}

dhallToNix "< Foo : {} >.Foo" = x: { Foo }: Foo x

Carefully note the difference between the 1st and 3rd examples. If the type of the alternative is present then you need to turn it into an anonymous function.

[ocharles]

Ah, so it's a case of < .. >.x being partially applied when the alternative does take data

[ocharles]

Ok, I believe my latest commit resolves this.

[ocharles]

Here's the latest behaviour:

[nix-shell:~/work/dhall-haskell/dhall-nix]$ ./dist/build/dhall-to-nix/dhall-to-nix <<< '<Foo : {} | Bar>.Foo'
x:
  { Bar, Foo }:
    Foo x

[nix-shell:~/work/dhall-haskell/dhall-nix]$ ./dist/build/dhall-to-nix/dhall-to-nix <<< '<Foo : {} | Bar>.Foo {=}'
{ Bar, Foo }:
  Foo {}

[nix-shell:~/work/dhall-haskell/dhall-nix]$ ./dist/build/dhall-to-nix/dhall-to-nix <<< '<Foo : {} | Bar>.Bar'
{ Bar, Foo }:
  Bar

[Gabriella439]

@ocharles: I think you need to import join: https://hydra.dhall-lang.org/build/10741/nixlog/2/tail

[ocharles]

Ah no, I just need to push another commit