Compatible, quoted syntax for YSH expressions

[andychu]

On #690, I would like something straightforward like:

a=(one two three)
if test -n "$BASH_VERSION"; else
  t=$(compgen hack)  # etc.
else
  var t = typeof(a)
fi

However Oil expressions won't parse in most shells. We would

So maybe we need something like:

a=(one two three)
if test -n "$BASH_VERSION"; else
  t=$(compgen hack)  # etc.
else
  t=${'typeof(a)'}  # avoid process substitution
fi

[andychu]

But then it could only return a string? How would you return an array and an associative array?

Maybe

t=${@'arrayfunc(x)'}

?

Possible trick: hide it behind a variable that can be defined?

$ if true; then t=${CALL-@'arrayfunc(x)'}; fi; echo $t
@arrayfunc(x)

If the script defines $CALL when running under bash, then it won't evaluate to the Oil expression string.

[andychu]

Better way:

$ echo ${OIL_VERSION+\$'x'} 
$ echo ${OIL_VERSION+\@'x'}

The quoted sigil \$ means return a string. The quoted sigil \@ means return an array.

Depends on #683

This has the nice property that you can use vars, double quoted, etc.

a=(one two three)
oil_expr='typeof(a)'
echo ${OIL_VERSION+\$"$oil_expr"}

Another possibility is to use single letter codes for readability:

echo ${OIL_VERSION+s'typeof(a)'}
echo ${OIL_VERSION+a'arrayfunc(x, y)'}

This doesn't disallow any possibilites because you can always quote it

echo ${OIL_VERSION+'s'}   # I want the letter s

---

note on quoted sigil evaluation:

$ echo ${OIL_VERSION-\$'x'}
$x
$ echo ${OIL_VERSION-\@'x'}
@x

[andychu]

I think this can mostly be done with eval