Re: newbee mystery

[Colin Yates]

I wouldn't say it is different, only that the REPL is doing an extra
step (printing) which forces the evaluation of that lazy sequence. As
James mentions, your code can force the evaluation of a lazy sequence
using doseq or dorun.

Lazy sequences are great when you get used to them but they can trip
people up, particularly in the fact that often a fn will return a lazy
seq which is then processed into another lazy seq which is then
processed into another lazy seq etc. etc. Finally it is evaluated and
an exception is thrown, and whilst the stack trace will contain the
call site that threw the exception it may be far away from the fn that
actually evaluates the seq.

You might also want to ensure that your REPL has a sensible value for
*print-length* (https://clojuredocs.org/clojure.core/*print-length*)
which stops the REPL evaluating the entire lazy sequence.

Without it, (range) will do nothing in your code but will blow up your
REPL :-).

On 9 October 2016 at 03:29, Phil Virgo  wrote:
> Thanks James.  So the repl behaves differently from some compiled code -
> good to know!
>
> I don't yet know about the "safe" issues "read-eval".  This came with the
> little template built by lein.  But I'll read up on it.
>
> On Saturday, October 8, 2016 at 10:11:29 PM UTC-4, James Reeves wrote:
>>
>>
>>
>> On 9 October 2016 at 03:00, Phil Virgo  wrote:
>>>
>>> WHY?
>>>
>>> The following prints 1/n 2/n 3/n fine in the repl
>>>
>>> (for [f [1 2 3]] (println f))
>>>
>>> But does nothing in this lein project (the "Hello World???" does print)
>>>
>>> (ns slide.core
>>>   (:gen-class))
>>>
>>> (defn -main
>>>   [& args]
>>>   ;; work around dangerous default behaviour in Clojure
>>>   (alter-var-root #'*read-eval* (constantly false))
>>> (for [f [1 2 3]] (println f))
>>>   (println "Hello, World??"))
>>
>>
>> The "for" macro is lazy. It only evaluates the items in the list when it's
>> consumed. You want the "doseq" macro instead, which is used for
>> side-effects.
>>
>> Incidentally, setting *read-eval* to false doesn't make clojure.core/read
>> safe. You should always consider clojure.core/read to be unsafe for use with
>> data from sources you don't control. Instead use something like
>> clojure.edn/read, which is designed to be safe.
>>
>> - James
>
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Re: newbee mystery

[Phil Virgo]

Thanks James.  So the repl behaves differently from some compiled code - 
good to know!

I don't yet know about the "safe" issues "read-eval".  This came with the 
little template built by lein.  But I'll read up on it.

On Saturday, October 8, 2016 at 10:11:29 PM UTC-4, James Reeves wrote:
>
>
>
> On 9 October 2016 at 03:00, Phil Virgo  
> wrote:
>
>> WHY?  
>>
>> The following prints 1/n 2/n 3/n fine in the repl 
>>
>> (for [f [1 2 3]] (println f))
>>
>> But does nothing in this lein project (the "Hello World???" does print)
>>
>> (ns slide.core
>>   (:gen-class))
>>
>> (defn -main
>>   [& args]
>>   ;; work around dangerous default behaviour in Clojure
>>   (alter-var-root #'*read-eval* (constantly false))
>> (for [f [1 2 3]] (println f))
>>   (println "Hello, World??"))
>>
>
> The "for" macro is lazy. It only evaluates the items in the list when it's 
> consumed. You want the "doseq" macro instead, which is used for 
> side-effects.
>
> Incidentally, setting *read-eval* to false doesn't make clojure.core/read 
> safe. You should always consider clojure.core/read to be unsafe for use 
> with data from sources you don't control. Instead use something like 
> clojure.edn/read, which is designed to be safe.
>
> - James
>

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Re: newbee mystery

[James Reeves]

On 9 October 2016 at 03:00, Phil Virgo  wrote:

> WHY?
>
> The following prints 1/n 2/n 3/n fine in the repl
>
> (for [f [1 2 3]] (println f))
>
> But does nothing in this lein project (the "Hello World???" does print)
>
> (ns slide.core
>   (:gen-class))
>
> (defn -main
>   [& args]
>   ;; work around dangerous default behaviour in Clojure
>   (alter-var-root #'*read-eval* (constantly false))
> (for [f [1 2 3]] (println f))
>   (println "Hello, World??"))
>

The "for" macro is lazy. It only evaluates the items in the list when it's
consumed. You want the "doseq" macro instead, which is used for
side-effects.

Incidentally, setting *read-eval* to false doesn't make clojure.core/read
safe. You should always consider clojure.core/read to be unsafe for use
with data from sources you don't control. Instead use something like
clojure.edn/read, which is designed to be safe.

- James

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newbee mystery

[Phil Virgo]

WHY?  

The following prints 1/n 2/n 3/n fine in the repl 

(for [f [1 2 3]] (println f))

But does nothing in this lein project (the "Hello World???" does print)

(ns slide.core
  (:gen-class))

(defn -main
  [& args]
  ;; work around dangerous default behaviour in Clojure
  (alter-var-root #'*read-eval* (constantly false))
(for [f [1 2 3]] (println f))
  (println "Hello, World??"))

(Thanks !)

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