> 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
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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