Strange stack bash error: --: command not found - bash

Question

I have very simple haskell project with only executable my-exec. All it does is printing "Hello, world!" to console.
I want to create the script file bin/setup.sh which will run my executable and also do some echo
#!/usr/bin/env stack
-- stack exec bash
echo Echo printing
my-exec
When I run it I get
$ ./bin/setup.sh
./bin/setup.sh: line 2: --: command not found
Echo printing
Hello, world!
And I don't understand what is the issue with this file and why it says --: command not found but still working as expected.
I understand that in this simple example I could write it in much easier form, but in my real situation I have to make like 10 of non trivial exec calls and don't want to duplicate stack exec multiple times.
So what can I do to get rid of this error?

Answer 1

Here's the problem. The first line:
#!/usr/bin/env stack
is interpreted by your operating system (e.g., the Linux kernel) as indicating that the script should be invoked using the equivalent of the shell command:
$ /usr/bin/env stack setup.sh
or, since env is just there to search the path for stack, the equivalent of:
$ stack setup.sh
If you run this manually, you'll get the same error. That's because, when stack is invoked this way, it reads the indicated file, searching for a line of the form:
-- stack blah blah whatever blah blah
after the first #! line. Normally, this line looks something like:
-- stack --resolver lts-10.0 script
which tells stack to run the script as if you had run the shell command:
$ stack --resolver lts-10.0 script hello.sh
which interprets hello.sh as a Haskell program, instead of a shell script, but runs it using the lts-10.0 resolver, and all is well.
However, you've told stack to use the command stack exec bash, so stack invokes your script with the equivalent of:
$ stack exec bash hello.sh
which is basically the same as running:
$ bash hello.sh
after setting up the stack paths and so on.
FINALLY, then, the shell bash is running your script. Bash ignores the first line, because it starts with # character that indicates a shell comment. But when Bash tries to interpret the second line, it's as if you entered the following command at the shell prompt:
$ -- stack exec bash
Bash looks for a program named -- to run with arguments stack exec bash, and you get an error message. The script keeps running, though, so the echo and my-exec lines get run as expected.
Wow.
Here's one way that may work for you. You can use:
#!/bin/bash
exec stack exec bash <<EOF
echo Echo printing
./hello
EOF
This shell script will invoke stack exec bash using a so-called "here doc", basically passing everything up to the EOF as a script file for stack exec bash to run.