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[Python-ideas] a in x or in y

Boris Borcic bborcic at gmail.com
Thu Feb 13 14:28:23 CET 2014 
Nick Coghlan wrote:
> On 13 February 2014 20:57, Steven D'Aprano <steve-iDnA/YwAAsAk+I/owrrOrA at public.gmane.org> wrote:
>> - given the restrictions on the parser, is this even possible? and
>
> I haven't worked through an actual candidate grammar (so I could
> potentially be wrong) but I'm pretty sure it requires more lookahead
> than Guido is willing to allow for the language definition.
>
> At the point we hit:
>
>      X in Y and
>
> we're going to want to process "X in Y" as an expression, and then
> start looking at compiling the RHS of the "and".
>
> This proposal would mean that when the "in" shows up after the "and",
> we need to backtrack, lift "X" out as its own expression, and then
> reinsert it as the LHS of *multiple* containment tests.
>
> It's worth noting that this deliberate "no more than one token
> lookahead" design constraint in the language syntax isn't just for the
> benefit of compiler writers: it's there for the benefit of human
> readers as well.
>
> In linguistics, there's a concept called "garden path" sentences -
> these are sentences where the first part is a grammatically coherent
> sentence, but by *adding more words to the end*, you change the
> meaning of words that appeared earlier.

Are you trying to say "X in Y and" forms a complete expression?? And couldn't the lexer recognize "and in" and 
cognates as single tokens anyway, like it apparently does presently for "is not" ?


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