cmd/--prefix: add --installed flag

[gromgit]

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This changes the output for uninstalled formulae from Cellar prefix to empty string, so:

$ brew --prefix abcde python@3.9 tcl-tk
/usr/local/Cellar/abcde/2.9.3_1
/usr/local/opt/python@3.9
/usr/local/Cellar/tcl-tk/8.6.10
$ brew --prefix --installed abcde python@3.9 tcl-tk

/usr/local/opt/python@3.9

$

Fixes #10261. I chose to print an empty string instead of not printing anything at all, because brew --prefix supports multiple formulae input, so this lets the user tell both visually and programmatically which formulae aren't installed.

For the same reason, "not installed" still returns success as before, so that "no such formula" remains the only error condition After a second think, since the switch is called --installed, it should return failure (to indicate that not everything requested is in place) but not an error message, since it's not an exceptional condition like "no such formula":

$ brew --prefix --installed abcde python@3.9 tlc-tk
Error: No available formula with the name "tlc-tk".
$ brew --prefix --installed abcde python@3.9 tcl-tk || echo "--> Returned failure."

/usr/local/opt/python@3.9

--> Returned failure.

[BrewTestBot]

Review period will end on 2021-01-11 at 11:45:07 UTC.

[MikeMcQuaid]

Nice work so far @gromgit!

[MikeMcQuaid]

$ brew --prefix --installed abcde python@3.9 tlc-tk
Error: No available formula with the name "tlc-tk".

Could even consider raising a NotAKegError here if it's not installed.

[gromgit]

$ brew --prefix --installed abcde python@3.9 tlc-tk
Error: No available formula with the name "tlc-tk".

Could even consider raising a NotAKegError here if it's not installed.

I'm not sure about that. "Formula not installed" is not an exceptional condition, and it's certainly not an unrecoverable one, unlike "no such formula". In fact, I can see a build script doing something like the following:

#!/usr/bin/env bash
shopt -s lastpipe
set -o pipefail
deps=(abcde python@3.9 tcl-tk)
prefixes=()

# Let's see what's already installed
brew --prefix --installed "${deps[@]}" | readarray -t prefixes
for n in "${!deps[@]}"; do
  if [[ -z "${prefixes[$n]}" ]]; then
    # Install what's missing
    brew install "${deps[$n]}"
  fi
done

# Get all the prefixes again, so we can use them for real
brew --prefix --installed "${deps[@]}" | readarray -t prefixes

[...]

[MikeMcQuaid]

I'm not sure about that. "Formula not installed" is not an exceptional condition, and it's certainly not an unrecoverable one, unlike "no such formula". In fact, I can see a build script doing something like the following:

It feels weird to return an non-zero status code and not say why, is my only concern. We get to define what this outputs/does as we're creating a new flag (and hiding it is only a 2>/dev/null away).

[gromgit]

It feels weird to return an non-zero status code and not say why, is my only concern.

Fair enough. I've altered the output to list the missing formulae on stderr:

$ brew --prefix --installed abcde python@3.9 tcl-tk 

/usr/local/opt/python@3.9

Warning: The following formulae are not installed:
abcde tcl-tk

$ brew --prefix --installed abcde python@3.9 tcl-tk 2>/dev/null || echo Missing stuff

/usr/local/opt/python@3.9

Missing stuff

[MikeMcQuaid]

Warning: The following formulae are not installed:
abcde tcl-tk

I feel like at that point NotAKegError is going to provide more consistent behaviour and output. Thoughts?

[gromgit]

I feel like at that point NotAKegError is going to provide more consistent behaviour and output. Thoughts?

You mean like:

      if args.installed? 
        missing_formulae = formulae.reject { |f| f.opt_prefix.exist? }
                                   .map(&:name)
        return if missing_formulae.empty?

        raise NotAKegError, <<~EOS
          The following formulae are not installed:
          #{missing_formulae.join(" ")}
        EOS
      end

which does this:

$ brew --prefix --installed abcde python@3.9 tcl-tk 

/usr/local/opt/python@3.9

Error: The following formulae are not installed:
abcde tcl-tk

? I don't have a strong opinion either way, but I'd rather respect existing semantics where possible, if only to make it easier for the next person to comprehend the code.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Thanks again @gromgit, great work here!