On Jan 30, 2013, at 5:59 PM, Michał Marczyk wrote:
> On 30 January 2013 23:32, Chas Emerick <[email protected]> wrote:
>> On Jan 30, 2013, at 12:23 PM, Michael Fogus wrote:
>>
>>>> RuntimeException EvalReader not allowed when *read-eval* is false.
>>>
>>> The problem is that the second eval gets (<the actual + function> 1 2
>>> 3) which invokes the right pathway triggering the exception. You can
>>> trigger the same exception by:
>>>
>>> (binding [*read-eval* false] (eval (list + 1 2 3)))
>>
>> Re-reading this, I'm clearly not grokking something here. Maybe I'm having
>> a slow afternoon; send help. :-P
>>
>> This obviously ends up running through EvalReader — but why? How is
>> LispReader ever involved at all?
>
> I believe the story goes like so:
>
> The eval call here compiles a list of a function object and three
> numbers. The function object gets compiled to code which effectively
> calls readString on "#=(clojure.core$_PLUS_. )". (It so happens that
> print-dup knows how to handle functions; if it didn't, an exception
> would be thrown during compilation with the message "Can't embed
> object in code, maybe print-dup not defined: ...".) When the compiled
> code is executed, readString gets called to reconstruct the function,
> and since *read-eval* is false, this fails.
Whoo, sneaky. If only fns carried (most of) the metadata that their
originating vars were defined with, print-dup would be able to emit a
fully-qualified symbol instead of that bonkers ctor call...I think.
This explains why my plan for a nuclear option for "fixing" *read-eval*'s
default doesn't work when outside of a bare `java -cp ... clojure.main` REPL:
https://gist.github.com/4674181
Much of the initialization of the nREPL / Leiningen / Reply toolchain involves
evaluating code that's been prn'ed, and there may very well be a couple of
nested `eval` usages lurking in there similar to what Fogus raised.
So, getting *read-eval* to a safe default is going to require more than just
setting its default to false; all usages of #= in
https://github.com/clojure/clojure/blob/master/src/clj/clojure/core_print.clj
need to be eliminated. Will be peeking at that next...
- Chas
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