& lifting above function call when it's part of the function

[bbrk24]

`a${&}b` c
// vvv
($) => `a${$}b`(c);

[edemaine]

The fundamental question here is whether & should lift above a function call when it's part of the function. Currently it does:

& x → $ => $(x)
&.simplify() x → $ => $.simplify()(x)

We definitely want e.g. &.toUpperCase() to lift above the call, so I think it makes sense.

Some of this might be related to ongoing discussion in #1450 about call chains, though that was for the . placeholder and this is about &.

If you need a way to stop lifting, I recommend do (though the output is pretty verbose here):

(do `a${&}b`) c
// vvv
(()=>{{ return $ => `a${$}b` }})()(c)

[STRd6]

I think this is the correct behavior. Another example to illustrate:

items.forEach .add 1
// items.forEach(($) => $.add(1));