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 😅

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

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.

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

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?

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