That looks workable. Does anyone have any more comments on '.?' versus '?.' ?
Joe On Tue, Apr 7, 2015 at 7:34 PM, Sebastian McKenzie <[email protected]> wrote: > No, you’d just memoise it to a variable: > > a?.d().f?.b > > to: > > var _temp, _temp2; > (a != undefined ? (temp = a.d() != undefined ? (_temp2 = _temp.f != > undefined ? _temp2.b : undefined) : undefined))); > > You’re going to need to memoise all member expressions anyway as they > might be accessing a getter and you don’t want to call it twice. > > > > > On Tue, Apr 7, 2015 at 7:32 PM, joe <[email protected]> wrote: > >> Okay. I've updated my transpiler to demonstrate how (and why) this could >> work at the VM level (I do think VM devs will have to add new opcodes for >> this), and I've appended an example transformation to this email. >> >> Why is this so complicationed? The answer is that when you start nesting >> ?. operators, it's pretty easy to cause some operators to be called twice. >> >> E.g, if you transformed the following to the NULL syntax: >> >> a?.d().f?.b >> >> You would get: >> (a != undefined ? (a.d() != undefined ? (a.d().f != undefined ? >> a.d().f.b : undefined) : undefined))); >> >> Notice how a.d() gets called multiple times. >> >> The solution is probably new VM opcodes. Since my transpiler is >> obviously not a VM, it transforms ?. operators into auto-generated >> functions. Anyway, here's the example I transpiler: >> >> a?.b.c.e?.f.g?.h.t.c?.d()?.e; >> >> Which turned into this: >> >> function q0eILlfx7_3(obj) { >> var _t=obj; >> >> if (_t==undefined) >> return undefined; >> _t = _t.b.c.e; >> >> if (_t==undefined) >> return undefined; >> _t = _t.f.g; >> >> if (_t==undefined) >> return undefined; >> _t = _t.h.t.c; >> >> if (_t==undefined) >> return undefined; >> _t = _t.d(); >> >> return _t; >> } >> >> q0eILlfx7_3(a); >> >> >> On Tue, Apr 7, 2015 at 6:24 PM, Brendan Eich <[email protected]> wrote: >> >>> joe wrote: >>> >>>> That's a good point. Are lexical non-DFA grammars allowed? It would >>>> be trivial to solve that with a regular expression lookahead. Although I >>>> suppose at that point you might as well call it a cover grammar. >>>> >>> >>> We must avoid being too clever -- it complicates implementations and >>> inevitably incurs future-hostility to extensions we may want, if not >>> outright bugginess. >>> >>> All of this suggests prefix ? is the better course. Anyone have >>> counterexamples? >>> >>> /be >>> >>>> >>>> Joe >>>> >>>> On Mon, Apr 6, 2015 at 2:42 PM, Brendan Eich <[email protected] >>>> <mailto:[email protected]>> wrote: >>>> >>>> joe wrote: >>>> >>>> By the way, I don't remember having grammar issues (I use a >>>> LALR compiler-compiler). Looking at my code, it looked like I >>>> handled it in the tokenizer stage; I added a COND_DOT token: >>>> >>>> COND_DOT : \?\. >>>> >>>> >>>> Did you keep backward compatibility? `x?.1:y` must continue to work. >>>> >>>> /be >>>> >>>> >>>> No, it looks like I didn't. >>>> >>> >> >
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