On Oct 26, 2011 7:15 PM, "Stuart Halloway" <stuart.hallo...@gmail.com>
wrote:
>>
>> checking out the "Try Clojure":
>>
>> if you type the following, you get output that matches what you typed
>> in every case except for lists.
>>
>> Vectors: --> [1 2 3 4]
>> [1 2 3 4]
>>
>> Maps: --> {:foo "bar" 3 4}
>> {:foo "bar" 3 4}
>>
>> Lists: --> '(1 2 3 4)
>> (1 2 3 4) <----- *INCONSISTENT* why not render this as '(1 2 3 4) ...
>> this would make much more sense to newbies.
>>
>> Sets: --> #{1 2 3 4}
>> #{1 2 3 4}
>
>
> This is an interesting question. Consistency is important, but consistency
with what? Your mental model for what happens at the REPL needs to keep the
R, E, and P steps clearly separate.
>
> Working backward: the P (Print) prints things in a way that they can be
read back, where possible:
>
> (read-string "[1 2 3 4]")
> => [1 2 3 4]
>
> (read-string "(1 2 3 4)")
> => (1 2 3 4)
>
> (read-string "#{1 2 3 4}")
> => #{1 2 3 4}
>
> (read-string "{1 2 3 4}")
> => {1 2, 3 4}
>
> If the P part of the REPL put a tick in front of lists, they would not
read back correctly:
>
> (read-string "'(1 2 3 4)")
> => (quote (1 2 3 4)) <---- INCONSISTENT
>
I see the problem that token " ' " is already taken to be short hand for
"quote" so to be truly consistent, I might be compelled to argue that
(read-string "'(1 2 3 4)") should, seeing some hypothetical "list start"
multi-char token " '( " .... just like set-start is " #{ ", be: => '(1 2 3
4).
You'd need two single quotes: "''(1 2 3 4) to get "quote '(1 2 3 4)" ...
Wheras (1 2 3 4) means "call 1 with ..."
I'll have to reread the explanation about the repl phases, but maybe I'm
wondering if things don't have to be quoted if lists had the syntax I
describe, and requests for function invocation had a simple paren. Then you
are not saying, "treat this as data, do not eval". You are just saying,
"see my single quote paren token? That's for lists".
Maybe it would be clearer if I proposed some other, lesser-used chars, like
"%(1 2 3 4)" or even "<1 2 3 4>". That is, I'm not so much saying, "this
needs to be treated as data and not eval'd" as I am simply saying, "this is
the 'list' data structure as opposed to some other".
Now, if you ever need to read in a program and treat it like "data", well,
what structure would you like to put it in? A list? A vector? A string? This
is, indeed, where I am fuzzy on lisps, but it seems like you parse it as
desired and put it where/how you like it. Why is there an assumption that
code as data means code as lists? Or is there?
> Now to the R (Reader) part. If, as you suggest, the tick were part of the
reader syntax for lists, you could "fix" the inconsistency above:
>
> ;; hypothetical Clojure with '(...) list literals
> (read-string "' '(1 2 3 4)")
> => (1 2 3 4)
>
> Finally, consider the poor E (Evaluator) in this scenario. The evaluator
must (1) evaluate lists except (2) *not* evaluate lists that are protected
by quote. But now that the tick is part of the syntax you have two
challenges:
>
> (1) Since the tick no longer prevents evaluation, you have to spell out
the quote (and still use the tick!)
>
> ;; hypothetical literal list
> (quote '(1 2 3))
>
> (2) Since the tick is required as part of list syntax, your programs have
to be written with ticks everywhere like this:
>
> '(defn hello-world
> []
> '(println "hello")
>
> You have to understand R, E and P as separate steps to understand the
REPL.
>
> As a side note, it is worth mentioning that the REPL is composed of three
simple parts, and that interactive shells that do not have this factoring
are much less useful. Imagine if your language didn't separate reading and
evaluating. You would need JSON or something to serialize data...
>
> Stu
>
>
> Stuart Halloway
> Clojure/core
> http://clojure.com
>
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