Output of "dhall freeze --cache" no longer works with Dhall v21.0.0

[joell]

The dhall freeze command has a --cache flag thats described purpose is to "Add fallback unprotected imports when using integrity checks purely for caching purposes". However, in dhall-haskell 1.40.1 the code generated from using this feature no longer works if the fetched URL content does not match the hash.

Example:

# observe how dhall freeze creates a cache-only fallback import for List/map
$ dhall freeze --cache <<< 'https://prelude.dhall-lang.org/List/map'
  https://prelude.dhall-lang.org/List/map
    sha256:dd845ffb4568d40327f2a817eb42d1c6138b929ca758d50bc33112ef3c885680
? https://prelude.dhall-lang.org/List/map

# submit the same code to Dhall but with a hash that does not match the content
$ echo '
  https://prelude.dhall-lang.org/List/map
    sha256:ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
? https://prelude.dhall-lang.org/List/map' | dhall
dhall: 
↳ https://prelude.dhall-lang.org/List/map
  sha256:ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff

Error: Import integrity check failed

Expected hash:

↳ ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff

Actual hash:

↳ dd845ffb4568d40327f2a817eb42d1c6138b929ca758d50bc33112ef3c885680


1│ https://prelude.dhall-lang.org/List/map sha256:ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff

(input):1:1

The expectation I had from the description of dhall freeze --check was that the ? operator should fallback from the integrity-check-failing import to the unprotected URL import and the expression should have succeeded (as long as https://prelude.dhall-lang.org/List/map could be loaded). Put simply, it seems this should work without emitting an error message.

Is dhall freeze --check's description out of date, dhall-haskell's implementation buggy, or my interpretation of all this faulty?

(Update: Given the breaking change to ? in Dhall v21.0.0, it seems the issue here is in the output of dhall freeze --cache not being revised to adapt to the breaking change.)

[joell]

I just found dhall-lang/dhall-lang#1181 in which the ? operator behavior was changed in a breaking way. So it appears that dhall's rejection of my incorrect-hash example correct to the v21 of standard.

However, that appears to leave the code generated by dhall freeze --cache failing to implement the functionality its --help description specifies.

Would it be sufficient to revise dhall freeze --cache to emit alternate code as follows?

$ dhall freeze --cache <<< 'https://prelude.dhall-lang.org/List/map'
  missing sha256:dd845ffb4568d40327f2a817eb42d1c6138b929ca758d50bc33112ef3c885680
? https://prelude.dhall-lang.org/List/map

This seems to satisfy the requirement that ? only "fall back to the second import if [the first import] cannot be retrieved". And it seems to work in practice.

[sjakobi]

Would it be sufficient to revise dhall freeze --cache to emit alternate code as follows?

$ dhall freeze --cache <<< 'https://prelude.dhall-lang.org/List/map'
  missing sha256:dd845ffb4568d40327f2a817eb42d1c6138b929ca758d50bc33112ef3c885680
? https://prelude.dhall-lang.org/List/map

That seems to fit with the documented intention of the freeze --cache feature.

@Gabriel439, what do you think?

[Gabriella439]

Fix is up here: #2350

[joell]

Thank you, @Gabriel439!

[Gabriella439]

You're welcome! 😊