`dhall format` removes required parentheses in freezed import statement using headers

[jgeerds]

This issue is related to #441. The parentheses around ./headers.dhall are required to make the code unambiguous. Unfortunately, the format command removes the parenthesis resulting that the expression no longer works

let MyPackage = https://server.test/package.dhall using (./headers.dhall) sha256:03a6e298ff140d430cea8b387fad886ce9f5bee24622c7d1102115cc08ed9cf8

dhall: 
↳ ./test.dhall
  ↳ ./headers.dhall
  sha256:03a6e298ff140d430cea8b387fad886ce9f5bee24622c7d1102115cc08ed9cf8

Error: Import integrity check failed

Expected hash:

↳ 03a6e298ff140d430cea8b387fad886ce9f5bee24622c7d1102115cc08ed9cf8

Actual hash:

↳ f251a7ed8ae08f533a11033e164afc73da09e65997a3f77d2345e34e42ea0f13


2│                                                                                                   ./headers.dhall
3│         sha256:03a6e298ff140d430cea8b387fad886ce9f5bee24622c7d1102115cc08ed9cf8

[sjakobi]

I think this is related to #1925 (comment). (Maybe @Gabriel439 can check!)

I suspect that there's no easy fix. I suggest that you workaround the issue by moving the headers import into a let-binding:

let headers = ./headers.dhall

let MyPackage = https://server.test/package.dhall using headers sha256:03a6e298ff140d430cea8b387fad886ce9f5bee24622c7d1102115cc08ed9cf8

[Gabriella439]

Actually, I think this is easy to fix. I believe all we have to do is to always parenthesize the argument to the using keyword. That was actually part of the original plan outlined here:

dhall-lang/dhall-lang#171

… which I just had never gotten around to following up on.

[Gabriella439]

Fix is up here: #2280