The general problem is that body blocks are not exactly blocks, due to
legacy cruft -- and this legacy cannot be separated from 'let' ideals
because we want programmers to refactor from 'var' to 'let'.
So we must give greater weight (compared to the ideal case of a body
block just being a block) to refactoring hazards that result in silent
but deadly bugs rather than early errors.
In particular,
function f(a) {
... // TDZ
let a = ...;
...
}
and
function g() {
try {
...
} catch (e) {
... // TDZ
let e = ...;
...
}
}
should be early errors because there's no useful shadowing going on with
'let' -- the TDZ means the outer binding cannot be used in the commented
places -- but any prior version using 'var' would have worked and
possibly allowed (coverage-dependent) uses of the "outer" (in catch's
case, var hoists across the catch head in sloppy code; in the parameter
case there's only ever one 'a' binding) use in the TDZ.
So an early error does no actual harm in either case, and helps avoid
bugs slipping past incomplete test coverage.
HTH,
/be
Brendan Eich wrote:
Andreas Rossberg wrote:
I think this subtle discrepancy is both unfortunate and unnecessary
[1]. Moreover, with ES7 do expressions, I would like it to hold that
(...) => {...} ≡ (...) => do {...}
I channeled you as best I could, and Dmitry Lomov kindly channeled you
on this point, but more than a few TC39ers objected that the left
arrow function, with a body block instead of a body expression, has
different semantics already, ignoring whether let x; in the body block
could shadow a parameter x. First, 'return' is the only way to return
a result in the left example, whereas thanks to do-expression being an
expression, the completion value (reformed) of the right ... is the
return value, even without 'return'.
I wanted to pass this back ASAP. More is being recorded in the meeting
notes, but here you go. We'll keep channeling you as best we can!
/be
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