Polyglot Interpreters

Question

Challenge

1. Choose any number of languages. Call this set L.
2. Write a program so that running it in any language in L produces an interpreter for a different language in L. Each language in L must have exactly one interpreter for it. This is to prevent, for example, Nothing or Text being part of L and all interpreters being for that single language.
3. Your score is the number of bytes of your code divided by the number of languages in L. Lowest score wins.

Rules

• Any language may have an interpreter for any other language -- it doesn't need to be one large cycle.
• An interpreter can input program code and input however you see fit.
• Standard loopholes apply.
• Different versions of a language count as distinct (e.g. Python 3.4 and 3.5) but not patch versions (e.g. Python 3.4.1 and 3.4.4). Thanks to caird coinheringaahing for this.

Example Submission (clarified due to Kaddath's comment)

# [Julia 1.0](https://julialang.org) and [Brainf***](https://esolangs.org/wiki/brainfuck), B bytes / 2 languages = B/2
```
#=
<julia interpreter written in brainf***>
[-][
=#
<brainf*** interpreter written in julia>
# ]
```
Julia 1.0: [Try it online!](https://tio.run/#julia1x)
Brainf***: [Try it online!](https://tio.run/#brainfuck)
Explanation:
When ran in Brainf***, the Brainf*** code is run. The `[-]` sets the current cell to zero, then we skip over the rest of the code using `[ ... ]` since we know the current cell is zero.
When ran in Julia, the `#= ... =#` marks a comment, letting the Brainf*** code not get executed. After that is the Brainf*** interpreter, then the final line would have an unmatched `]`, so it is commented out.

Answer 1

Jelly u / Python 3 + jelly, 70 bytes = 35

from jelly import*
a=sys.argv
main(a[1],[*map(try_eval,a[2:])],'')
ŒV

Try it online! (Jelly)

Try it online! (Python)

Both error after executing the other language.

This is a bit of a cheat tbh. Jelly has an "execute as Python" builtin, ŒV, and, as of April 9, 2018, Jelly is a pip-installable package, meaning that we can just import it and run it.

How they work

Jelly ignores every line except for the last, and just runs that:

ŒV - Main link. Takes a string on the left
ŒV - Execute the string as Python code

For some reason, I think because of the [...] in the above lines, there's some parsing error thrown afterwards.

For Python, we import the contents of the jelly package, read and evaluate the command-line arguments, then run the program in the first command-line argument with the values in the second and third arguments

Answer 2

Windows Batch/Powershell 7, 38 bytes, 19 score

ls >$null&&cmd /c "$args"
pwsh -c "%*"

Assumes that there is no file named ls in current directory plus Powershell 7 executable pwsh.exe is in PATH.

It needs Powershell 7 because && and || syntactic sugar was introduced in that version.

It first checks if any command is ls, >$null suppresses the output. If it succeeded we assume the interpreter is Powershell, so we execute Batch code and vice versa.

It generates some extra errors besides the STDOUT, but I think that's fine because @caird's answer also errors.

Answer 3

Windows Batch / PHP < 8, 25 bytes, Score: 12.5

php -r %1;echo`$argv[1]`;

(cannot really) Try it online for PHP! (it runs, but external scripts seem to be disabled, for security reasons I guess)

Cannot test Windows Batch in TIO and haven't found another site for it (makes sense actually) -> you can place the code in a test.bat file and run something like test.bat "echo 42;" to pass the PHP code as parameter

• Assumes that PHP executable is in PATH under the php command.
• in PHP below version 8, php and r are assumed string constants, and - % are operators, this part tries to calculate the result but does nothing with it. The rest of the code executes the argument as command-line code and outputs the result.
• in Windows Batch, executes the first argument received as PHP code and outputs the result. The rest of the code is considered as a second argument and is ignored.