Add support for Unix shebangs

[Gabriella439]

… as standardized in dhall-lang/dhall-lang#1158

[TristanCacqueray]

nit: could this be replaced by a <- endOfLine ?

[basile-henry]

It would be nice to support multiple lines of shebang so that we could use the nix-shell shebangs.

I have just tested this branch with the following (from the root of the dhall-haskell repo):

#! /usr/bin/env nix-shell
#! nix-shell -i "dhall --file" -p "(import ./.).dhall"

42

Which currently gives me:

➜ ./test.dhall
dhall:
Error: Invalid input

./test.dhall:2:1:
  |
2 | #! nix-shell -i "dhall --file" -p "(import ./.).dhall"
  | ^^^^^^^^
unexpected "#! nix-s"
expecting expression or whitespace

I saw the dhall-lang PR for this, but didn't think about mentioning the nix-shell shebang use case at the time 😅

[Gabriella439]

@basile-henry: Yeah, that would first require an upstream change to the standard, which we can still do afterwards

[TristanCacqueray]

It seems like nix-shell shebang is relying on the fact that # is a comment in the target language, but that does not always work. Though that seems like a good thing to support.

Alternatively, perhaps nix-shell could be modified to also consider --! nix-shell -i "dhall --file" -p dhall as a valid extra shebang line, but that would not look good.

[TristanCacqueray]

Also it is unfortunate that #!/usr/bin/env -S nix-shell -p dhall -i "dhall --file" does not work.

[basile-henry]

nix-shell shebang is relying on the fact that # is a comment in the target language

That's probably true for bash and python. However I have seen multi-line shebangs with nix-shell for Haskell too, usually with GHC's runhaskell. That probably means that GHC takes this edge case specifically into account?

[TristanCacqueray]

runhaskell does something special where it ignores any lines starting with '#!', not just the first ones, according to: https://gitlab.haskell.org/ghc/ghc/-/blob/e61d2d47c4942c829ec98016be5aa5ae36982524/compiler/GHC/Parser/Lexer.x#L279