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Creating a mock REPL on Unix

Category: misc

During the development of REPL.nvim I had to be able to test the plugin without relying on any particular REPL present on the development system. The solution was to create a mock REPL, a shell script which acts like a really dumb REPL. Here is the code:

EOF=false

echo 'This is a dummy REPL, it does nothing and comes with no warranty.'
until $EOF ; do
        printf '>>> '
        read -r || EOF='true'
        if [ ! "$REPLY" ]; then continue; fi
        echo "$REPLY"
done
# Terminating new line if there was no reply
if [ ! "$REPLY" ]; then echo ''; fi

The first line is a simple state variable which will be set to true once the user presses ^D. As long as the variable remains false the loop will keep running.

Next comes the actual Loop part of the REPL; it prints a prompt and then reads a line indefinitely. The Read part is implemented using the built-in read to store the result in the implicit variable $REPLY. Should read read ^D the exist code will be non-zero, causing the conditional || to set the EOF variable to true. This is our exit mechanism out of the REPL. There is no Evaluate part since this is a mock REPL (or you could say that any input evaluates to itself). Finally the Print part is implemented by using echo to echo back the input.

Technically the REPL could also be implemented with less code:

EOF=false
until $EOF ; do
        printf '>>> '
        read -r || EOF='true'
        echo "$REPLY"
done

The additional code is just to give the REPL some extra polish. It's not really needed if no human will never use it, but I like the extra touch. The first line

if [ ! "$REPLY" ]; then continue; fi

prevents the loop from reaching the echo if $REPLY is an empty string. Without it if the user does not enter any text an empty line would be printed:

# Without extra touch
>>> foo
foo
>>>

>>> bar
bar

# With textra touch
>>> foo
foo
>>>
>>> bar
bar

The other line

if [ ! "$REPLY" ]; then echo ''; fi

causes echo to display an empty string. This empty string will move the shell prompt onto the next line:

# Without extra polish
sh-3.2$ sh mock-repl.sh
This is a dummy REPL, it does nothing and comes with no warranty.
>>> foo
foo
>>> sh-3.2$

# With extra polish
sh-3.2$ sh mock-repl.sh
This is a dummy REPL, it does nothing and comes with no warranty.
>>> foo
foo
>>> ^D
sh-3.2$

The mock REPL can then be invoked like any other shell script and be used in place of a real REPL program when testing. That way we do not have to be wary of any side effects and the developer does not need to have any particular program installed on their system.