On Tuesday, December 9, 2014 11:42:40 PM UTC-5, Ryan Schmitt wrote: > > It turns out there are some subtle issues which can cause incorrect >>> behavior were clojure.core/read to blindly wrap a PushbackReader around its >>> argument: >>> https://groups.google.com/d/msg/clojure/_tuypjr2M_A/W1EcEbMUg_cJ >>> >> >> That sounds like magic. The user wrapping a PushbackReader around a >> BufferedReader doesn't cause problems, but the library function doing so >> does? Why would where the wrapping takes place make a difference? Is the >> *only* problem the rare case of reading more than one object from the same >> stream? A docstring warning to wrap manually *in those cases* would >> suffice, then, no? >> > > It's not a question of *where* the wrapping takes place, it's a question > of who holds a reference to the wrapped Reader and what their expectations > are. The reason that clojure.core/read cannot wrap its argument in a > PushbackReader is that it doesn't know who else holds a reference to that > argument. Let's say I have a BufferedReader that initially contains the > following two Edn elements: > > "string1""string2" > > Let's also say that I have a version of clojure.edn/read that accepts a > BufferedReader and (unsoundly) wraps it in a PushbackReader. After the > first read call, the PushbackReader clojure.core/read created will contain > the following: > > "string2" > > But the BufferedReader might only contain the following: > > string2" > > Since the PushbackReader created by clojure.core/read will be garbage > collected after the function returns, it will be impossible to parse any > more data correctly out of this reader. And this won't even break all of > the time--it will only *occasionally* break, in the cases where a > non-whitespace element gets pushed back into the PushbackReader and then > garbage collected. > > As for the motivation behind using a PushbackReader at all: > http://en.wikipedia.org/wiki/Parsing#Lookahead >
If that's the only issue, then there's not really a problem as long as read accepts a PushbackReader and doesn't further wrap one if it is given one. Then the user in an instance like the above can do their own wrapping *when they must*, but can also eschew it when it won't be a problem because they intend to discard the whole stream after the one call to read. -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
